Pros: Comfort, light weight, spectacular sound: response and imaging
Cons: Not cheap
I've used most of the old-school brands (Grado, AKG, Sennheiser, Sony, Koss, David Clark, ...) and have tried several of the Jonny-come-latelys, and the HD700s are simply the best for comfort and sound that I know.
I can wear them for hours and never get fatigued.
Most of the others that I've heard fail relative to the HD700 in terms of either wearability or audiophile sound.
I use an after-market cable that's shorter and lighter than the stock cable.
My other options are Etymotic fitted in-ear 'phones or Genelec active monitors.
I've never heard the HD800s with anything better than a computer (and a PC at that) driving them, so I can't really compare the two.
I listen mostly to computer audio (Apple Lossless or AAC@320kbps formats) through a TC Electronic Impact Twin FireWire DAC/Headphone amp, which I also highly recommend (esp. for the price of $250).
I've used lots of AKG 'phones over the years, and liked the sound, but the Q701 is noticeably heavier than the HD700 and flops around more when you move your head, the Senns are snugger and lighter, in addition to having a killer soundstage.