They're rather comfy once you lightly stretch the heaphones out a little bit. For $100, they're probably the beat headphones you can get under $200.
Having heard both, and now that they're cheap, I'd say the AD900's are better, but the M50's are probably one of the best headphones under $100 (My vote is still the fostex T50, but that's just personal preference)
Good deal on a great headphone. I only tried these for a short while, so I can't comment on comfort, but I have a small head and didn't see any issue.