I've worn the M50. I found they clamped but not nearly as bad as some other closed headphones like the Shure 440 and Sony MDR V6.
Sound quality I would rate as pretty good. A little too "bassy" for my tastes. Probably why it is a darling on headfi.
Anyways, this is a good writeup on this headphone.http://www.innerfidelity.com/content/audio-technica-ath-m50