Pros: Bass is quite good, headphone seals well
Cons: Harsh and grating highs, high clamping force.
A friend picked up the ATH-M50s as his first decent headphone, and we spent a couple of days swapping back and forth between them and my DT770 Pro 80ohm.
I found the M50s to be quite well built considering how much plastic was on them. They felt solid, and the folding mechanisms were smooth and felt quite good.
The pads were very soft, and the pleather felt rather nice and sealed very well. The headband, however, was not padded very well, and along with the high clamping force of the M50s, lead to comfort issues.
I would describe the M50s as having a very V-shaped sound signature.
The bass extension, quality and quantity easily competes with my DT770s (although this could be because the M50s seal better, I think if I put (p)leather pads on my Beyers they would get a fairly significant bass increase).
The mids are quite recessed, and maybe a bit thin sounding. Very detailed though, when not overshadowed by the other two ends of the spectrum.
The highs are what ruined this headphone for me. Listening to Pendulum's "Showdown", the cymbals in the first 30 seconds as well as any S sound from the singer made me cringe. They came off as very harsh, especially in combination with the lack of mids. I may just be more susceptible to highs than most people, but I really couldn't enjoy much of my electronica or any music with a lot of high frequencies without a pretty significant EQ.
After a week, my friend brought the M50s back and got some DT770 Pro 250ohm instead, which we both preferred much more. These could be decent headphone for around $100-$120, especially in regards to build quality, but certainly not for their current price of ~$150, in my opinion.