Some non-fanboy feedback.
1st of all I didn't buy the m100's a friend of mine did and i happened to be able to listen to them, compared directly to the audio technica m50's and my meelectronics m6's (which i carry my sandisk clip zip and headphones in the little carrying case, can't be beat).
The sound signature of the m50's and the m100's seem very close to me. I couldn't tell a discernable difference between the two other than a very slight extension on the bass from the m50's which appeared to drown out the mids at times.
The detailing and sound stage of the m100's is vastly superior. It literally sounds like you are actually there. The m50's and the meelectronics are close, but the muddling they receive in complex music makes me wish for more.
The m50's have better isolation for sound, the m100's aren't bad but they do have some sound leak, but not much.
Size - the m100's are tiny when folded up for an over ear headphone. id say they take up about 25% less space than the m50's.
My friend listened to the m100's for about 30 minutes, then packed up his m50's and, said merry christmas (after talking so highly about them for the last 6 months).
Songs listened to:
Blackest Eyes - Porcupine Tree (m50's received vocal distortion during the first chorus approx, 1:20 into the song, the m100's or the m6's didn't)
Don't Worry Child - Swedish House Mafia (sounded great on all, but the m100's just sounded a lot clearer)
Random other house/edm songs, some rap.
Overall if i had $300 bucks i would purchase the m100's, they are great headphones. However, i barely make minimum wage and will be content with the m50's as a good headphone i happened to get for free (many thanks to my friend). In other words, if you have the m50's and want a boost in clarity and range, and are looking for an upgrade, the m100's are a very similar headphone. Sorry i don't have much else to compare to,