Pros: Bass, Isolation, Warm sound signature (if you're into that), Easy to drive, Cost, Included accessories
Cons: Sometimes a little sibilance, Not as airy as open headphones, Non-Detachable cable
I ordered the AT-M50s for work less than a week ago. I wanted something easy on the wallet that wouldn't require amplification beyond my uDAC-2. My coworker found a $95 special on SlickDeals and I couldn't resist. I have owned a pair of AT-M40 for a long time prior to my head-fi days and SQ obsession, and I used to think they were "nice". After getting HD-650s and pairing them with Bifrost/Asgard 2 I realized the HD650s were in a different universe of clarity, soundstage and overall SQ. I decided to give Audio Technica another chance however.
Great bass, muddy compared to the HD650, treble harshness. For the first few hours I wasn't to happy with the way they sounded, but hey they were still burning in. I kept listening. After 8 hours or so, the sound got noticeably better. Treble harshness had faded away, and the headphones were starting to grow on me. Moreover, they sound decent amped my iPhone alone, my uDAC-2, and the Schiit stack I use. This cannot be said for the HD650 I can contrast them against. The AT-M50s are fun to listen to, and have great isolation. After doing an A/B comparison I feel the sound quality is definitely at the level of the HD650, just different in nature. The HD650 has them beat on airiness and natural sound, but really only quite subtly so.
In conclusion: These headphones cost 1/5th as much as the HD650 (my only real point of reference), have nice build quality, are comfortable, and sound great being driven from a variety of sources. I would highly suggest these to anyone looking to get into HiFi headphones without spending an arm and a leg. Also for electronic music, rap, and other bass-heavy music these cans really do have excellent bass, much more so than the HD-650. I wouldn't say the bass is overpowering or overly EQ'd like Beats, just powerful and present.