Pros: Excellent bass, great impact and body, surprisingly comfortable for large noggins.
Cons: Lack of treble resolution, a bit of ringing, some quality control issues, heavy, hot.
Disclaimer: My opinions are just my own. They do seem to be in contrast to some others' experiences, but we all have different preferences.
I heard great things about this headphone, and really wanted to like it. I can see why they are popular, and they indeed are fun. However, I'm a treble-head, and as one who was spoiled by the beyerdynamic DT880's, I hated them.
(TL;DR: I bought these new from Headroom. Compared to the beyerdynamic DT880's, the HiFiMan's have much, much better bass impact, a fuller body on all frequencies, and less sibilance. The DT880's crush the HE-500's when it comes to treble resolution, and a slight lead in mid and low frequency resolution.)
(Off topic: I have not the slightest clue why beyerdynamic insists on using all lowercase letters, but this isn't a typo.)
Before we start: my setup was a Benchmark DAC1, feeding into a Meier Corda Opera. Files were lossless, fed bit-perfect from iTunes to a M2Tech HiFace (with a regulated, linear power supply). Cables included double-shielded power cables, a triple-shielded USB cable, all interconnects used shielded Mogami wire and Neutrik terminations with matching impedances. Primary comparison was with a beyerdynamic DT880 (600 ohm, with a Blue Dragon cable, whose drivers were broken in with over 300 hours of pink noise- even before I began using them a year before). Power conditioning was with a medical-equipment/mission-critical-telecom grade double-conversion, fully online, UPS. (That's just how I do things!)
On to the listening tests:
I burned them in with pink noise for about 100 hours before giving them a serious listen. The first impressions was that there was fantastic bass impact. These are fun headphones! The mids had decent resolution with excellent body, and the bass was not flabby. Stock, everything was a bit veiled and the bass was a tiny bit muddy, but that was resolved by removing the foam damping sheet, as recommended by many owners. I'd highly recommend trying that modification to see if you like it.
Compared to the DT880's, if you like bass, then there is no contest- the HE-500's win by a mile. (Then again, that isn't exactly a hard thing to do.) Additionally, the HiFiMan's had more body to the mid and low frequencies, and there was less sibilance than the beyerdynamics have. Some people find the DT880's habit of brutally reproducing sibilance and recording flaws to be fatiguing, others don't. I am in the latter group. Note that the HE-500's resolution improved dramatically with all frequencies when I removed the internal foam damping sheet. I didn't notice the "tizzyness" some people complain about.
The ugly... coming from beyerdynamic DT880's and being a treble-head, I found the high frequencies seriously (in comparison to my fantastic treble-loving beyer's) lacking resolution. The HE-500's treble resolution was miles behind the beyerdynamic's, and the mids on the HE-500's were slightly less precise than the DT880's. Smooth and friendly treble, yes. Resolving- not all that much.
For example, with the beyerdynamic's and violin recordings, I can easily tell which way the bow is moving, and the slight "crunch" the bow hairs can make on impact with the strings during intense sections. . With vocals, I can hear when the performers take a breath. This all makes me feel more involved and intimate with the performers, especially as a violin player. The HiFiMan's had very little of that, and I missed the intimacy and detail.
There was also a slight "shouty-ness" with the HiFiMan's. Testing has shown that there is a bit of a ring in the 1kHz area, probably due to poorly designed housings. That bothered me a lot, but then again, take this with a grain of salt as I am very sensitive to it. (Fun fact: designing good housings is very difficult- just see how hard Sennheiser worked to reduce the resonance and ringing when they designed the HD800's, so it is understandable that HiFiMan, as a smaller company, might not be able to invest in exotic plastics and crazy geometric designs like Sennheiser did.)
In the end, just make sure you take my own preferences into account. These all could be good characteristics for some people who want something more forgiving or more impactful.
Initial impressions were that they seemed solidly built. There was no play/slackness in the pieces- fit adjustments were easy and crisp. I also really appreciated the inclusion of both a balanced and unbalanced cable, and a set of spare terminations for their RF-originated cable attachments (absolutely nothing wrong with that, but it is a bizarre design choice.) The spare part for DIY cables was a thoughtful touch which impressed me.
However, most of the unboxing experience was bad. Though the box was of nice quality, the headphones had a very strong smell of volatile organic vapours (AKA, cheap plastic smell) and a week of airing out later, the smell was still strong on the headband- which showed that the packing material was not to blame. (They really need to rethink that headband material.) Given others' experiences, I suspect they have since resolved that smell issue. In the end, though, this is just not acceptable for $700 headphones.
The 1/4" TRS cable was poorly made on my particular set- the 1/4 inch termination was already loose, and there were bits of plastic hanging off where the mould lines were. The XLR cable was of okay ("meh") quality. It was functional, and since it was, my tests were used the balanced cable and with a nice, not cheap, XLR to TRS adapter, given the compromised build quality on the TRS cable. This affair is not something I even see on most $30 headphones. It made me suspect that, at least on the day mine were made, there were some quality control issues with the cables. It's not a huge thing as most will probably just throw these out and use nicer aftermarket cables, but then again, this is an expensive piece of kit, and I can't forgive the lack of consideration.
And yes, these were purchased new from Headroom.
In the end, I didn't like them, and returned them (Headroom has fantastic customer support, by the way!) The sound wasn't bad, per say, but I couldn't take the lack of treble detail and the slight kHz level shout-y ringing/resonance problems (which were present with and without the damping foam). They wouldn't necessarily be bad as a second pair of headphones for me, and if someone gifted them to me, I'd keep them. If you want something quite fun and with a lot of very controlled kick (AKA, what my anthropomorphised beyer's would, with a haughty attitude, grumpily call "plebian" and then say "Bah, you're all Philistines!") while still having decent detail retrieval, they are fantastic.
Don't forget to try removing the foam damping sheet. You might like the difference.