Pros: Crisp, clear detailed sound, with excellent (not excessive) bass
Cons: Not the most comfortable phone for extended listening.
After scouring the Head-Fi forums, and noticing that HeadRoom.com included the ATH-M50S in their "ten best" list with the next best headphone at a much higher price point, I pulled the trigger on these for $110 new from an eBay seller that was a large professional music equipment dealer (an excellent source for good prices on headphones apparently, because they carry the full manufacturer's line - including microphones - I have a pair of new Sennheiser 650s coming from another pro music audio dealer that I got for $319), and have been overjoyed with the ATH-50S coupled to a tiny NuForce uDac-2 HP ($99 headphone-only DAC-amp) ever since.
I am new to Head-Fi and digital audio generally, so I can't compare the M50 to any other headphones, but based on my long-ago experience selling really high-end audio, they are like experiencing the sound of an excellent pair of full-range, expensive loudspeakers, except they take up a lot less space, are more portable, cost much less for a comparable degree of sound quality, and allow private listening. I really needed a closed-back headphone because I listen in a lot of circumstances where I can't disturb those around me, and the ATH-M50S are excellent at containing sound "leakage" while not being as insulating from allowing you to hear outside sounds (like fire alarms, doorbells, etc.) as an IEM would be. I listen to the the M50s in the public library at what seems like a really robust volume to me, and people sitting just a few feet away from me never glare or look askance at me. They are easy to drive (I can actually play them quite acceptably from my tiny Rockboxed Sansa Clip+, and the disparity of size between the source and the cans and sound eminating from them is kind of amusing. With the Nuforce uDAC-2 HP serving as a replacement for the internal sound card in three different notebook computers (a Sony i7 running Windows 7, an old HP dual-core and an Asus Atom netbook - both running Ubuntu Linux), they can be driven to deafening levels without a hint of distortion at any frequency, even what we used to call "bass doubling" coming from speaker woofer cones breaking up from the cones not moving linearly trying to push large volumes of air quickly. Another advantage of headphones with a small enclosed acoustic cavity over trying to fill a room with high-quality sound.
The closed-back design does lead to a smaller soundstage, which is why I have the Sennheiser 650s coming, which I will use only at home where I can afford to "bleed" some sound to the area around me. The only real complaint I can make about the M50s is comfort. The build quality is excellent, and even though high quality plastic is used to keep them relatively lightweight, they are still a tad heavy, and the clamping force is strong, but tolerable. Helps for a good acoustic seal, so I would be wary of trying to stretch out the headband too much.
Supply and demand tells you a lot about the quality of any product. When I got my pair for $110 new on eBay, the Amazon price was about $129. Now, they are going for almost the full retail of $199 on Amazon, and the number of five-star reviews of them grows daily as people take delivery.
Definitely a keeper.