Pros: Bass, Mids, Highs, Soundstage, Solid Construction, Supreme Comfort
Cons: Needs better cable to excel, paint job a little fragile
What is there to say about the new flagship Sennheiser that hasn't already been said over the past year. With that in mind, I will give my impressions about this headphone and why I believe that most people that don't like it haven't listened to it properly amped and for long enough time. This is a headphone that easily wows for the first few minutes you try it on....often blowing away any other headphones you've heard before. However after that initial honeymoon is over, many are very bothered by the highs, lack of bass, large soundstage on certain kinds of music, and being overly analytical. To me, the HD800 is very much of a Chameleon, like the Stax SR-007 Omega, meaning that even though they are easily amped, they are extremely revealing of the power, source, cables, amp, etc. and will drastically change in sound signature depending on what equipment you have them hooked up to.
As other have said, these are probably the biggest and most comfortable headphones you'll try on. They are so airy that you don't feel that your ears are enclosed inside the cup of a headphone. The construction is impeccable with perfect weight distribution and styling to match. The cable / connectors are very well designed for a stock cable and reeks of quality. I upgraded mine eventually and found there to be positive benefits. Otherwise, the paint can get slightly dinged up over time, but that is nitpicking.
These headphones are ruthless (much like the Stax O2) and have caused me to upgrade my source to a Perfectwave DAC and my amp to a Woo Audio 5. Even then I wasn't content and had to roll in a bunch of tubes until I found the ones that had the best definition, bass, and impact. Because of the difficult nature of these, I've had a love hate relationship with them but believe that I've recently built my system such that I'm extremely happy with them with almost all music I listen to. I have tried the HD650, Denon D7000, Beyerdynamic T1, Hifiman HE-5, and Stax Omega 2 hoping that they will displace my HD800s, but at the end none of those were good enough in my eyes to keep.
For people that own HD650s, you will be in for quite a change of sound. Gone is the Sennheiser veil and the enclosed soundstage, but the mids in my eyes are pretty close to the signature Sennheiser sound. When not amped properly, these will sound thin, sibilant, with too much instrument separation. When properly amped with an amp that has enough power and has a sound signature that will complement the HD800s, they will have the deepest bass you will hear (if the song is meant to have bass), wonderful instrument separation and soundstage, and beautiful highs without being sibilant or shrill. Because they have such an amazing dynamic range, songs are rendered beautifully, with good timbre, and with a ton of impact. Songs that have acoustic passages like jazz or guitars are the best I've ever experienced by far. Granted they are not bassy like the D7000s, but in my eyes those are overly bassy even on songs / passages that aren't meant to be. On a good system play "Hyper-Ballad" by Bjork and you will be amazed by how deep these headphone can go, its astonishing!
With all of this said, I highly recommend these headphones only if you are willing to invest in the rest of your system to make these shine. There are easier headphones out there such as the T1, but in my book the HD800 rewards extremely well as your system progresses up. I also suggest that you don't look into too many people's impressions of these based of a few hours of listening because they require a whole lot more time to adjust to and fully appreciate. As for price, based on the recent trend of headphone flagship pricing (Audeze LCD-2, Stax O2, AKG K1000, Beyer T1, W5000) I think they are very appropriately priced for their amazing technical proficiency that I find unrivaled, especially in the dynamic headphone arena. Good job Sennheiser!! Its pretty obvious that you have spend countless years of R&D to make these headphones just right!!