Pros: Comfortable, detailed, big soundstage
Cons: Artificial sounding, anemic in the bass, can be too bright, overpriced
I have listened to the HD800 twice, and I never really warmed up to it. Its clarity and resolution sounds artificial to me instead of natural (a spike in the upper mids region), and it has no authority in the sub-bass region. I'm one of those people who simply cannot consider a pair of headphones to be "amazing" or "the best of" if it's lacking neutrality in a chunk of the frequency range.
A amazing pair of headphones should sound like a full-range speaker system that reaches down to 30Hz and remains substantial and authoritative--anything less than that is not "amazing" to me. Now, pardon me for turning into a pig for a moment and fall back on the classic but eyebrow-raising comparison to a woman. It's sort of like if a girl is really hot with an awesome body, but her ass is flat, barely able to fill any pair of jeans--would that still be considered an amazing body? (This comparison is actually quite fitting in a humorous way, since low frequency in audio is often referred to as the "bottom-end.") Even the HD650 has more sub-bass extension and weight, and it costs far less than the flagship model.
I understand that there's a portion of people whose idea of neutral bass is in fact anemic bass to me, but most people have no idea what a neutral frequency range sounds like, because they have never heard true full-range sound before. Anyone who's ever heard a full-range speaker system that reaches down to 30Hz or lower while maintaining ± 3 dB, will know that neutral bass in in fact quite authoritative and substantial.
There are headphones out there that can reach down low and feel very authoritative--for example, the Audeze LCD-2 and LCD-3, Stax 009, 007MKII, Denon AH-D7000, D5000, D2000, Audio-Technica ATH-M50, etc, so it's not like the HD800 is somehow limited by physics--it was a choice the engineers at Sennheiser made.
If the lack of full sub-bass was the only issue, I'd have been fine with the HD800, but it is also overtly bright in the upper mid-range, which can be shrill/sibilant on some material, and that breaks my number one rule of audio: "First, do no harm." When any audio gear produces sound that is too bright, it becomes grating and it hurts your ears, and when that happens, it's a deal breaker for me.
Many defenders of this attribute of the HD800 will go to lengths to remedy the problem by buying stupidly expensive headphone amps or other unnecessary audio gadgets to tame that brightness, and they would proclaim that if one used a sufficiently high-end tube amp, the HD800 will sound much better. Really? It appears the marketing department of high-end audio gear companies are doing a damn fine job selling absurd diminishing returns. A pair of headphones is not supposed to have inherent problems that needs to be fixed with yet another piece of expensive gear in the first place. If someone tried to pull that in the professional audio world, they'd get laughed out of the marketplace. This isn't to say there aren't too-bright sounding professional monitor speakers, but at least they were designed with onboard EQ's and measuring mics to adjust according to the room acoustics. If you want to alter the sonic signature of any audio gear--use an actual EQ, not an expensive amp used like a single-preset EQ.
Some people say the HD800 is very revealing, like a sonic microscope. Well, so were the Yamahama NS10's--the legendary monitor speakers that's dominated the pro audio world for decades, but they were used only in the context of being a mixing/mastering tool, and only for troubleshooting potential problems. No one uses them for leisurely listening or a balanced overall presentation, because they were too bright and lacked authoritative sub-bass. If you're not using the HD800 in that way and are listening for pleasure, I think you can find aural bliss in another pair of high-end headphones that doesn't do as much harm and has a more full-range sound.