I don't have enough experience with other Senns so I can't compare the HD800 to them. I prefer the HD800 over all my other headphones (K340, HP2, HF2). I think that Tyll described them well. They get out of the way of the music. They don't have wow factor, they just deliver the music as delivered from the source and amp. I can listen to it for hours with no fatigue and no discomfort.
To me it's what everybody says they want in music reproduction. I guess a lot don't really want it.