OK...here's a few impressions of the HD800:
We listened to them with a variety of gear, including a Wadia 830 (CD and iTransport), and a HeadRoom Micro DAC fed via PC as sources, and a RSA Raptor, TTVJ Portable, and Gilmore Light as amps. We had as a basis for comparison an AKG K701, and Sennheiser HD600 and HD650.
To summarize: I like them...a lot! I'm going to have to buy my own pair.
They have a sonic signature that is unlike pretty much any other dynamic headphone that I've heard. If pushed, I'd say that they were similar to the AKG K340 when modded well, with elements of the K1000 mids thrown in for good measure...but better. I associate what I had heard more with electrostats than anything. Their response was lightning quick and effortless. Highs were brilliant without being harsh or distorted, and the midrange was rich and had genuine depth. Soundstaging was simply out of this world, especially when listening to a binaural recording (it sounded like there was a clarinet 20 feet behind me to the right).
The bass response was, frankly, the most interesting thing to me. It was absolutely there, and lightning fast and impactful. That said, its not like I could feel the HD800 thumping on my head while I listened. I listened to a couple of tracks known for their bass (DMB's "Don't Drink the Water" and Patricia Barber's version of "Use Me") and it sounded perfectly natural. I never once felt the HD800 vibrating or otherwise resonating. Jude explained to us that Sennheiser took great care to ensure that the materials used in the construction of the frame of the HD800 were accoustically neutral, and that explains why music with heavy bass doesn't shake your head, but it sure is there!
They are also, without question, the most comfortable headphones that I've ever worn. They are light, and the earcups sat pefectly on my head. It's almost like they weren't even there.
Are they the perfect headphone? Well, I don't know that there IS such a thing. Having said that, in my short time spent with these cans I can't find much of anything where they come up short. Are they worth the asking price of $1400? Frankly, I'd have to say yes. I was that impressed.