No two heads are alike, so your mileage may vary. I like circumaural headphones, and within this genre I thought the Sennheiser HD 800s were quite comfortable for long periods. The special alcantara material on the pads is very nice. The pressure is a little high, but the pads are so big this is well distributed.
A truly excellent headphone, the HD 800 will appeal to those who want a smooth and balanced sound with ample but not punishing clarity.
Sennheiser HD 800: Another View, an updated and expanded version of this review (thanks to the discussion here) can be found in the forums.
Additional commentary and thoughts also can be found in the Ask It: Sennheiser HD 800 in the Headphone Forum.
You may wish to read coverage of competing designs:
Sennheiser HD 800 Headphones
Weight: 330 grams
Impedance: 300 ohms
Ancillary equipment used: Esoteric DV-60 universal player, Lector CDP-7 CD player, Musical Fidelity V-DAC, Benchmark DAC 1 Pre, Luxman P200 headphone amp, PS Audio GCHA headphone amp, Head-Direct EF1 headphone amp, Woo Audio WA 22 prototype headphone amp, Nordost cable