The HD800s are ridiculously good, offering an ultra-revealing, speakerlike presentation one step away from K1000s, neutral tonal balance, earth-shattering low end, wicked, precise highs... I find them to be very dynamic as well. The high end is a departure from the HD600/650 sound and is both the greatest strength and weakness of the 800s as it's simply too revealing for a lot of folks.
I find them to be foot-tapping at low volumes (55-65dB) and mind blowing at moderate to high volumes (80-90dB). At higher volumes I get a 'live music' feel and they make incredibly good cinema phones / gaming phones, offering pinpoint imaging. Imaging on regular headphones can sound a little blurry even with crossfeed, but that's generally minimized on 800s.
The 800s can be fatiguing at high volumes with recordings that have a boosted high end (listen to trance written by Above & Beyond and your ears are going to feel like they got stuck in an oven), but are absolutely incredible with a neutral recording of pop/electronic (The Crystal Method - Vegas), and are great reference phones for classical recordings, especially piano (Barenboim's Chopin Nocturnes).
Jazz lovers should consider auditioning the 800s. Whether it's due to the woven steel backends, unique new driver, or driver properties, they reproduce drums and metal instruments - vibrophones, cymbals, brush drums - much better than HD650s, which sound on the dull side in A/B comparisons.
My recommendation would be to listen to the HD800s before buying - they present a new sound that die-hard Sennheiser fans may or may not like, trumping the Sennheiser Orpheus for detail and offering what I would compare to other Sennheiser high-end phones a slightly experimental tonal balance - it just doesn't sound rounded at the lows and highs, sort of a "you guys do nothing but whine about the veil, well... eat this, kids" statement. You may like it or hate it - don't go out on a limb and buy before trying!! I did my due dilligence and listened carefully to them before buying, too.
Bassheads will delight in the 56mm drivers: buzz-free and fully extended without sacrificing articulation down to monstrous sub-bass lows - on some (bass-heavy) recordings it feels like the subwoofer is strapped to your head, it's a noggin tickling experience which shockingly doesn't distort the midrange or high end... you can't get this sound by EQing HD650s, which are already bass kings. People looking for headphones capable of reproducing the RumbleFX effect without sacrificing other aspects of the sound could EQ the HD800s instead of buying a subwoofer to compliment another set of phones like K701s, HD600s, etc.
Try the HD800 with a headroom amp and DAC - they really shine with a neutral, articulate amp and source. They sound terrible with a lot of equipment out there - unlike the 650s or DT250-80s they are source, amp, and recording dependent, more so than HD600s, a real "tell it like it is" headphone. I loved my HD650s through a source that sounds awful with 800s.
For reference purposes, I'm a pretty big fan of the Sennheiser sound, but also like and respect the DT250-80s, K701, DT880, Sennheiser orpheus, and can understand why people would like Grados, though I don't prefer them. Give the 800s a shot if your tastes are similar to mine and you have a neutral to slightly warm sounding amp / source combo.