If you have waded across the web all the way to this review, I'll assume you've already read some of the big forum threads on this site, and I'm sure you've already read all of the reviews on other websites and publications about this piece of gear, so you already come into this with some background knowledge about this device and devices like (or unlike) it.  
  
I have the Benchmark DAC1 USB: the second model slightly upgraded from the original design of the now classic Benchmark DAC1. Before this, I had (and still have) a Nuforce Icon HD. I use both of these as both DAC and headphone amplifier for my Denon AH-D5000, and preamp for my KRK studio monitors.   
  
I'll try to be brief and avoid much colorful, interpretive flights of language that you see all the time in audiophile reviews.  
  
Sound quality: noticeably better than the Nuforce Icon HD. Volume balance between channels is perfect at all levels. Noise floor is noticeably quieter. Bass is a little quicker and more "real." Things seem just a little more clear in the mids and highs are nicer and less fatiguing/stressful at higher volumes. The Icon HD sounds fantastic to everyone who listens to it, and this I would say is a tier above that. Difference is not life and death unless you are exceptionally picky, but there is a difference and it is very good. There is no coloration to this sound - it is just accurate, clear and strong. Use an EQ with this if you want coloration, or look elsewhere for technically same/worse performing devices that come colored off the shelf.
  
Features, inputs, outputs: I can use this with pretty much any device that puts out a digital audio signal, and I can plug into this pretty much anything that accepts an analog signal. I can toggle between inputs with a really cool little switch, and one of the two headphone outputs is capable of muting all rear outputs when it is occupied. A switch on the back can change the XLR and RCA outputs from volume controlled (with the very nice volume knob) to flat, constant line level. Attenuators (-0,-10,-20,-30) for the rear outputs can be adjusted as well. There is no power button, it just turns on and off very effectively depending on whether or not you're using it. This sounds strange and untrustworthy, but no - it works great in practice. There is no display, but it doesn't need one. "It just works." Feed it a signal and give it something to output to, and that's genuinely all you need to worry about, marvelously simple design.
  
Build quality: this thing is heavy, sturdy, good looking, and sensible. It is large enough to be impressive but not so large that it is inconvenient. It does not feel like it will break, ever. Gets warm to the touch after use. Friends agree that it looks "awesome."   
  
This is an exceptionally clear, high-performing and versatile device in a well-made package. If you want a good DAC/amp to control your powered speakers, and/or a good DAC/amp to power your headphones, this is the device of devices. It is simple, it works - and it works really, really well.  
  
From the charts I've seen, you really don't get gear much better than this. You can get devices that color things differently or look physically different, but this is pure neutral pro audio right here and it is as good as any piece of audio gear needs to be. Everything competing against this or existing at a higher bracket than this is, in my opinion: either about prestige or niche coloration. You can get a device that rolls off the highs or uses unique sounding parts to make an exotic but nice sound that is somewhere east or west of "accurate" if you want - or you can get 100% pure sound right here with the Benchmark DAC1. Once you get to this level, things are strictly about personal preference, because price/performance becomes meaningless when performance plateaus and money isn't an object anyway.
  
 

This is exactly like the $100 more expensive Icon HDP. I'll quote the description on Nuforce's website: "The Icon HD is designed for users who want the Icon HDP's world-class USB DAC and headphone-amp performance without having a need for the Icon HDP's flexibilities" 
 
It is an HDP stripped down to a usb input, a DAC, the RCA output, the volume control, and the 6.3mm headphone output. For a person desiring only a DAC/amp for their headphones and nothing more, this is a much better deal than the HDP which might have unnecessary (depending on user) inputs and outputs and features.  
  
This is my first "real" amplifier. So, I cannot say a whole lot (please refer to the Nuforce Icon HDP reviews, as whatever is true for that is true for the HD as well, as the dac and amplifier sections on each are identical), but I can compare it to what I am used to.  
  
Using it with Denon AH-D5000 and Sennheiser HD595 headphones, it sounds fantastic. I cannot determine any flaws. As others have said, there is no noise floor and this puts out great power for a full sound (no missing bass, no muddied highs, etc). It's exactly what I would expect a hi-fi amplifier to be.  
  
Comparing it to my other amplification solutions, this thing is a giant above any of it. It beats the output on the HT Omega Claro Plus+ sound card in power, bass, and shockingly to me, even clarity - something I thought my sound card particularly excelled in. This is crystal clear compared to the Claro, which, while clear itself, just isn't quite as transparent as the Nuforce. Speaking of clarity and transparent qualities, the Icon HD completely wrecks every headphone output I've ever heard on any SOny/Pioneer/JVC/Denon stereo receiver I've owned or used. I haven't heard any super hi-fi receivers; only the more affordable ones, so the Icon's superiority is definitely expected and understandable. 
 
I think a lot of people are content with their "cheap" amplification. They just plug into their sound card, or they just jack into their home theater receiver, and they call it a day. That is what I did for the past 2 years. Today, I realize that I have been missing something that makes a huge difference. I strongly encourage anyone without discrete headphone amplification to go out and get this - at only $350, and with all of the quality of the HDP, this is a steal. 

These headphones are alarmingly great.  
  
Now, I want to first talk a little about myself. I am not seasoned veteran of Hi-fi. I do not have any experience with multi-thousand dollar listening setups. These are simultaneously the most expensive and greatest headphones I've owned. Of course, their quality is overwhelming to me. Please read this review with the understanding that I might not be "qualified" to accurately or usefully compare these headphones to similarly priced models of other brands. 
  
I come from Sennheiser HD595 headphones. Before that, I used Audio Technica ATH-A700 headphones. These are both excellent choices. Both of these headphones wowed me when I got them. Prior to the ATH-A700s, I had never heard "good" audio, period. I currently run my Denon AH-D5000s through a very modest stereo amplifier hooked up to the line out of an HT Omega Claro Plus+ sound card, playing ASIO-outputted flac stuff. I would venture that this is a very adequate setup for music and media listening, although I am sure it is not directly comparable to the setups that some people have spent $1000+ on (between DAC and amp).   
  
Anyway, modest context of my experience aside, I want to start to discuss the headphone itself, the Denon AH-D5000.
 
 
First, we have the package. The box is not special. It is slightly reinforced with an interior plastic frame (so I think the delivery guys and warehouse guys would have to seriously mistreat it in order to damage it), but it is not special. I would have liked a slightly better case, but that is not very important. Removing the equipment from the case is an easy affair that takes only a few seconds - open the box, slide out the plastic frame, untwist some ties, lift out the headphones. It is simple and straight-forward.  
 
The first thing I notice is the weight and appearance. The headphones are very light, but they feel remarkably sturdy. The ATH-A700, with their complicated headband apparatus (you know what I mean) were light, but also seemed a tad flimsy. The Sennheiser HD595 seemed more sturdy than that, but the Denons are in a completely different realm of craftmanship. This is the difference that several hundred dollars can make.  
  
The headband is simple black leather (or fake leather, whatever). The earpads are the same. The frame is said to be magnesium - I suppose this is the reason for the apparent stability. The cups can extend downward from the headband, as most headphones can do. I was impressed, though: the headphones extend downward by the mechanism of notched metal bars - even for headphones this expensive, I was expecting more plastic parts.   
  
Plastic parts are not what this is about. The headphones feel beautiful, and they are beautiful. The first aesthetic element you are likely to notice will be those gleaming, polished mahogany cups. I knew these would look great, but, having never held wooden-cupped headphones before, I was pretty happy just to look at these and enjoy their appearance. These things look exactly how I would imagine "nice" headphones to look - classy, simple, modern, sleek, interesting and shiny. The cord is also nice - I have heard of people complain about the cord before, but I wonder how they are judging it, or to what they are comparing it. The cord is wrapped in a woven-thread/cloth sheath-thing (please forgive my utter lack of articulation), and it is thick and good. It is not a cheap and simple rubber cable you would expect from a $10 set of headphones; it is (at least ostensibly) a very nice cable. It is flexible and the length is acceptable for my purposes. 
  
When plugging the headphones into my receiver, I of course had to attach the 1/4 plug over top the "default" 1/8 plug. This mechanism is also very smooth and pleasing, both to use and to look at. Shiny and delicate threads allow the adapter to snugly and perfectly screw into place on the edge of the plug, finally coming into its position as if it were a natural extension of the cable. My other headphones had adapters, sure, but they were not this nice. They were all simple - pop it into place, listen for a slight click and it's in. I'm not sure how much the screw-mechanism helps the sound of the headphones (it probably does not), but it definitely smells of quality.   
  
As far as sound goes (perhaps the most important thing about headphones, right?) I am not sure I am up to the task of describing it. I read other reviews of the AH-D5000 before listening to the phones myself (indeed, before even purchasing them), and I know a few of the things they say and tend to agree with some of those things.  
  
First, the mids. Everyone talks about the mids, mentioning that they seem recessed. I will agree that the mids feel a bit "distant." The bass is very "up front" and the highs are also very apparent, but the mids feel like they are slightly further away. I do not think this is a bad thing. I don't know if it is a good thing, but I do know that it is an interesting sound and I suppose the fact that the mids feel a little further away helps the overall sound to be more distinct, with separated and easily distinguishable parts. Everything is very distinguishable, and every sound has a place. Again, I feel the need to apologize for my woefully inadequate articulation on these points. What I want to say is that every sound I hear, I can easily single out and hear on its own, unblemished by the other sounds. I suspect that the "distant" middle range of sounds helps create this effect.  
  
Next, the bass is incredible. I would call myself a basshead, as I listen to a variety of electronic music that even includes some dubstep (well-known genre for uncanny amounts of bass). For Trance, Drum & Bass, and Dubstep (and a few other subgenres of electronic) these headphones are absolutely wonderful. The specifications say that these phones do 5Hz all the way up to 45000Hz -- I am definitely getting the low end of my Armin Van Buuren and such on these cans. I do not think it is "flabby," -- when I think of "flabby" bass, I think of people with car stereo systems with the bass-boost on, and the dashboard is vibrating and the bass is shaking things so much that it produces a sound resembling flatulence. I do not feel this effect in the Denon AH-D5000s. Amplification might affect this - I do not know. In any case, I feel that the bass is great. There is certainly a lot of bass (far more than my other headphones), and the bass sounds more controlled, tight and clean than the subwoofer I have for my speaker system (although that is admittedly mid-fi and I'm sure there are FAR better subwoofers out there than what I have). In short, the lows are great here.  
  
The highs, then, are the last specific thing I can discuss about the sound. Piano music sounds exceptionally crisp and clear. I have heard people in other reviews discuss sibilance, and harsh sounds that come from the bright highs of these headphones. I know that my cheapest earbuds I use for my Zune that I bring to work produce all kinds of horrifying hissing, staticy and vulgar, grating noises when the high notes are hit by singers and guitars and the like. I even get vague bits of this sometimes (very rarely) with my HD595s. I am just not experiencing this at all with the Denons, though. Highs feel smooth. I mean, they are not jarring, they are not ear-piercing, nothing unpleasant like that. They sound clear and beautiful.  
  
Speaking of awful noises, I suppose I can also discuss "listener fatigue." I had no such thing with the Audio Technicas, but I would experience some fatigue with the Sennheisers. After a few hours of listening, my ears would feel hot and uncomfortable, pressured and unpleasant. If I were to pause the music at that point and listen to silence, I would hear a minor ringing. I don't think this was the fault of the Sennheisers (but perhaps it was the fault of the particular combination of Sennheiser + my stereo). Now, the Denons do not give this effect to me at all. I feel like I could leave these in for hours and hours and hours and not mind at all.   
  
Music sounds wonderful. As I've already mentioned, electronic music absolutely shines. I also listen to some alt-rock like the Cardigans, Boa, the Cranberries - and they sound great, too. The bright quality of these headphones is fabulous for the guitars and keyboards of such a band. The singers all sound so defined, too. While on the topic of singers, Sarah Brightman and Loreena McKennit both sound fantastic, and back to rock-related things, I've found these headphones perfectly useful for enjoying Pink Floyd, King Crimson and the like. Also, these headphones can resolve all of the chaos going on in some pretty intense death metal like Cryptopsy's None So Vile. The layers and instruments are far more distinguishable here than they were on other phones. Some classical music that I occasionally enjoy also sounds pretty great, but I am not enough of an avid, active listener of classical to feel like I can say much about it, other than that I think it sounds great on the Denons, just like the other stuff does. In short, I like all of my music on these more than I liked it on my other headphones - but I have to repeat, electronic music REALLY shines on the Denons, even moreso than anything else.  Stuff like Bassnectar (with a huge variety of sound), sounds just unbelievable. Who says the bass is overpowering?
 
 
Well, that's about all I can say. I might add an update when I feel these have more completely burned in, but that probably will not be necessary. I'm sure they'll still sound great. I know it's the writing of a pretty inexperienced Head-Fier, and I could stand to hear a lot more headphones so that I can better compare them to each other - but this is it for now. I like these a lot, and I hope you can see where I am coming from and why I like them as much as I do. I suggest these to everyone, but primarily I suggest them to anyone who favors electronic music. I feel that this type of music is the strong point of the headphone here.

I bought these in London over the last summer and have used them as my primary music-listening device since then. They are plugged snugly into a value-level Pioneer A/V receiver which is attached to my PC, playing a variety of FLAC electronic, rock, metal and pop music.   
  
 
It is hard to point to something that these do that I feel is absolutely excellent. Things are clear, nothing's really muffled. If there's a flaw, I feel like the bass is lacking in these phones, but that doesn't mean there is no bass at all - they certainly have enough to make listening to trance/D&B/Dubstep enjoyable.   
  
However, they are definitely lacking in sound quality. I feel like they are a very meager upgrade over the ATH-A700s I had previously. Of course, most high end audio is a world of meager upgrades. I am looking to upgrade in the near future, though, hoping that another more expensive set of headphones will be more "wowing." 

I had these and used these as my primary method of enjoying music, films and video games for 18 months until I replaced them with a more expensive model of a different brand. Before I owned these, my "greatest headphones" were $40 VOIP headsets. I won't even begin to claim I have golden ears or anything like that, but even a philistine like me could tell that these things were a million miles ahead of any game headset or stock Zune earbuds around. It was so much better than my previous equipment that I actually experienced the "I'm hearing things I never knew existed in my music" effect. This is to be expected from anyone's first pair of good headphones, though.   
  
They sit on my head extremely comfortably. I would notice that I was wearing headphones, of course, but it is the same sort of way that you might notice you are wearing a very familiar hat that you've worn every day for years - you know you're wearing these headphones; it is hard to forget that they are on - but they are not at all obtrusive or unpleasant to wear and you absolutely have no objection to wearing them for an entire day if you have the freedom to just sit down for an entire day. However, with excessive head movement, you can hear the slightly adjustable,swiveling parts of the headphones creaking just a tad, so it's not always a good idea to "rock out" when wearing these. This is the one flaw I can consider when thinking of these headphones - not that I'm sitting here headbanging or anything, but even slight head movements are enough to trip the little creaky noises.   
  
Again, they sound great. I listened to trance (Armin Van Buuren, Tiesto, ATB, etc) other electronic (various Drum & Bass, ambient, Aphex Twin, etc), rock (Pink Floyd, The Cardigans, King Crimson), metal (King Diamond, Dismember, Emperor, Blind Guardian, Death), and a little bit of classical stuff (Mozart, Vivaldi). I am not experienced in a diverse array of headphones, so I don't know much about how other headphones might be more suited for specific genres, but I enjoyed all of this music and more on these headphones.  
  
They don't require much amplification. I say "much," because they sounded kinda "ghostly" when played directly through my first-gen Zune. Bass was very undefined, and things were quiet in strange ways. Plugging into an HT OMEGA PLUS+ sound card worked beautifully, though, and the headphone output of a variety of stereo and A/V receivers worked beautifully, too. A $200 home theater receiver has a good enough headphone jack to make these sound pretty good. I'm sure the cheap dedicated headphone amps you can get work well, too.   
  
These are very affordable at only around $110 or so today. For the price, they are absolutely great. However; they aren't perfect. Other headphones are even clearer, other headphones are even more comfortable, other headphones have crisper vocals and tighter bass. These aren't bad at all and I cannot point you to any obvious flaws in their sound or feel or construction. Just, there are obviously better headphones out there beyond these. I consider these an excellent gateway into audiophilia.