There have been a few of these threads, and we're pretty burned out on them I reckon, but I think David nailed with his comments pretty well. I've thrown in my comments in the thread, as have other some of the other, well-known "usual suspects", so it mostly makes for good reading.
Edit: I managed to get many of my thoughts together about the HD-800s in a post in the thread:
I think I have an idea why.
Firstly, let me say I've owned a pair since the week they were released to the public, I even invested in a second pair when word spread that different pairs have different sound signatures. I didn't notice a true difference between the two pairs I owned, except that one was just a hair brighter in the upper mids, but we're not talking anything that one could notice unless purposefully listening through several tests. I then sold one of the two pairs on head-fi.
Anyway, I want to comment on the headphones I currently own, not for bragging purposes, but because I want you, the reader, to have a clear conception of what my experiences are.
The most prestigious headphone I own is the R10 (bass heavy), I have also owned the bass light and I sold that one as it didn't thrill me as much. After that, the best headphones I have owned are the Ultrasone Edition8, HifiMan HE-5 and several other headphones which would probably be deemed mid-fi by the incredibly high standard listed above (HD650 / HD600 / K701 / DT880 / DT990 / SA5000 / D5000 / DT770)..........I'm not still in posession of all those mid-fi cans, but I do still own about half mentioned.
When it comes to reproducing acoustic instruments I have never yet heard a headphone capable of the realism which the HD800 is capable of. This includes the R10s, I feel the HD800 is the most true sounding headphone when reproducing un-electronically filtered instruments. For classical music, and most jazz (collectively consuming about 70% of my listening time) I prefer the HD800 about 90% of the time. I think there is a misconception that they are bright. If I had to call one headphone truly neutral and truly "uncolored", I would refer to the HD800 as choice number 1. There are certainly things which the R10 and HE5 are capable of doing which the HD800 are not. For one, the R10 makes nearly everything sound good! The R10s are lush and beautiful like an impressionistic painting. You want to live in that world, but it's not the world you truly live in. I would still say in many ways the R10s are the best sounding headphone I've heard because they really do make everything sound special. But I do consider the R10s (bass heavy) to be a warm headphone and possibly even a colored headphone in that I find it's bass impact to be more forward than neutral. The HE-5 is an amazing bargain (however as of the date of this OP I hear they are still working out some defects on specific pairs).....the HE-5 has a very close resemblance to the R10s in terms of response. The HE-5 differs in that it is slightly more aggressive, has a distinct drop in bass impact and is eversoslightly less transparent than the R10, but for $600 versus $6000 (the average going rate on a good pair of R10s these days), I would say the HE-5 are close to a steal.
But then there's the HD800 which transfer sound to my ear in way that is simply different than all other headphones. It's not the size of the soundstage or the very meticulous imaging abilities which I think make the HD800 stand apart. I'm going to mention something which I think has never been said and it's very important to me....
In headphones, when sound leaves the driver and travels to you ear it is creating the illusion that there is a sound source somewhere around you. When bass frequencies hit your ear, you can actually feel the vibration in addition to hearing it. I have not yet heard a headphone until the HD800 which was able to create the illusion that bass source I was hearing was actually centered rather than feeling a hard left and hard right bass source. Even if the bass is centered in the mix, the notes of the bass are center, but the vibrating resonances which you can actually feel in your ear have this feeling (at least for me) of being left/right, not centered.....this flaw for me has always subtracted from the experience of headphone enjoyment. People have remarked that the bass on the HD800 is lacking, but for me, it is the ONLY headphone I have heard which truly shows how bass should be done in a headphone. Firstly, most music is mastered today to be listened to through "fun" headphones or speakers. I would say that most commercial mastering today is bright as a 300 watt light bulb and much of it truly lacks bass extension. So when you're listening to a song which has been mastered like this, it is absolutely going to make the HD800 sound bright. However if you take out a modern classical recording or an ECM Jazz recording (I trust the way they master their music) or a Steve Hoffman DCC or Mofi CD, I assure you the bass is there. Firstly, the HD800 has the deepest bass I have ever heard. The impact is large in that the notes are very present, but the impact of feeling the bass against your ear is less than usual. This creates the illusion that you are not hearing bass, but in fact you are, all the way down to 20 hertz. When it comes to rock music I do not recommend this headphone as your main one, it will not have the impact in the bass region necessary for getting you deep into the music, but if you are interested in hearing acoustic instruments sound as natural as possible I believe these are worth the $1400 price tag.
I think the thing which makes the HD800 as polarized as it has become is that it's not a pretty sounding headphone. It doesn't manipulate the music to sound good. In fact some songs and entire recordings sound awful on them! Does this mean it's a "niche" headphone? Possibly...but I don't think a headphone can truly be great without being a "niche" headphone because it is my opinion that due to the varied way in which music is mastered and mixed, no headphone can ever perform equally well in all genres while still outclassing other niche headphones of specific genres.
I don't think the HD800 are better than the R10 necessarily.......the R10 make a lot of music sound better than reality if that makes any sense?
If I were to sum up the HD800 in one analogical sentence it would be this:
If you're willing to hear the truth, and you're feeding the headphones truth (unfiltered recordings), you will hear as much of the truth as headphones can currently offer, it is up to you to appreciate this truth, or prefer a prettier distortion of it:-)
I've noticed as one heads towards the high-end and the soundstage grows, instruments become more separate and distinct and one moves father and father away from the all-mushed-together sound of lo-fi gear, in some respects it's harder to find a combination of components that delivers the music in a way that's enjoyable, as instead of a wall of sound, without all the distortion, music can easily start to sound boring. This is why, I think a lot of people get the "My iBuds sound better" comment when trying to introduce friends to high quality gear. They just aren't prepared for the music to come from individual instruments with "black" between them instead of a wall of sound blurred together. The HD-800s take this further ahead again, by removing more of the "wall", which with many headphones is created with a mid-bass boost. This is also why people (including myself) felt the need to re-cable them with cables that add just a touch more bass, as we want some of that warmth back, that is, the mid-bass boost part of the "wall" we're used to.
Listening, to, say, Alicia Keys as she sings in a recording studio with a degree of resolution that one can visualise the size of the booth she was standing in from the echos off the walls, with some very poor crap laid over her vocals is just too much info. Many of us like to have something to fill in the "black" with a lot of music, and hide from us the cables running across the floor of the recording studio, imagining instead we're listening to an overwhelming live performance. This is arguably more so if the music is mostly electronic.
On the other hand, there is music where we want to focus on the intensity of the individual performers so that the spirit of their performance is transferred to us as much as possible. This is where the HD-800 excels, up to the degree of the component chain's ability to do the same. However, despite this, it can end up leaving us a bit further back in the audience than is ideal to get the intensity we might crave with some music. This is much the same as putting jumbo/bagel pads on some Grados -- even if it means everything spaces out, you lose the intensity to a degree, and lose some of the "wall", and some of the fun with certain music.
The result is an annoyingly higher demand for extracting great aural enjoyment from them. Even Stax are more forgiving in this regard. Instead of bringing more fun (which, since I was quoting rhythmdevils, he might call the lack of a "flaw", though there are many technical aspects to this from their frequency response to their impedance curve we could find fault in if we looked at it objectively rather than subjectively), we get more resolve, so if not more resolution of the music, more resolution of the capabilities of the components feeding it. We're back to the Head-fi newbie fault of buying good headphones and a cheap amp and sourcing it from our cheap computer sound-card again, but much further up the scale. And we're back to square one. We either buy something that tonally balances them or we buy a lot better gear, depending on what kind of enjoyment we're looking for when listening to music.
I hope this rant made some sort of sense.