Pros: Excellent value, substantial sub-bass , non-fatiguing, relatively neutral/accurate, folds for traveling
Cons: Pleather gets sweaty, rotating earcups annoying if you're not a DJ, soundstage a bit small, not for those after perfectly neutral frequency response
(Disclaimer: This review was written while considering the very low price-point of the M50, and what you can get for that amount of money. It does not mean the M50 can go up against the high-end headphones that cost several hundred to thousands of dollars. My main headphones are high-end headphones, and the M50 is only used while I'm doing tracking or traveling. I wouldn't use it as my everyday headphone since I have superior headphones for that, such as the Audez'e LCD-2.)
The ATH-M50 is one of those rare products where the quality/price ratio really hits the sweet spot, and in fact is like a small miracle in the world of pro audio. When you get Grammy Award-winning audio engineers and producers like George Massenburg, Frank Filipetti, Al Schmitt...etc singing its praises publicly, you know it's got to be something special. (Though let's be honest--those guys probably wouldn't mix on the M50, although they'd do tracking on them.)
The M50 pulls off the difficult balance of being neutral, accurate, and detailed while not causing listening fatigue, and that is one of the most important things to get right when it comes to any audio device. If the device hurts your ears with shrill or piercing treble, then no matter how "detailed and revealing" you think it is, you won't be able to withstand the sonic torture anyway. Designed as professional studio monitors, the M50 can be used all day long without any listening fatigue, and it's tonal balance is accurate enough that many respected audio engineers would not hesitate to do tracking with them (though mixing on them is probably asking a bit much). Being sealed headphones, they also are a favorite among musicians and singers when recording, as they do not bleed into the microphone like open headphones (which means you also won't bother the people sitting near you, unlike open headphones where others will hear a tinny version of what you're hearing).
One of the most outstanding characteristics of the M50 is its sub-bass extension (50Hz and below), which is both deep and substantial. The quantity of bass is slightly more than neutral, so it's a good headphone for those who likes a bit more bass than neutral. Its raised bass is a broad and gentle curve and sounds quite natural and pleasing, and is not annoyingly bloated or distorted.
The mids and the treble are smooth, and the treble never gets gratings like many other headphones. If I must nitpick, I might say that the treble is slight hard and has a metallic timbre when compared to open-back headphones. But it doesn't get in the way of the music too much and it's only noticeable if you do A/B comparisons with headphones that have very smooth/neutral treble response and know intimately how specific instruments are supposed to sound (such as the cymbals on a drum kit). What I really love about the M50's upper mids and treble is that it follows the rule of "First, do no harm." With other headphones that "fake" detail by raising a few to several dB's in the lower treble/high-mids region, some songs can sound very shrill and fatiguing--especially on sharp snare hits or vocal sibilance--but on the M50, those tracks sound quite balanced and natural, never harsh and irritating.
The soundstage of the M50 is smaller than the average open-cans, because of its sealed design. This is perhaps the only thing sonically I wish it could be improved upon, but this does not mean the soundstage of the M50 is claustrophobic or in any way detrimental to the listening experience--it's simply not as open and lush as headphones like the Sennheiser HD6XX/5XX series (and other high quality open-cans).
Physically, the M50 is pretty comfortable to wear, but pleather tends to get a bit sweaty, and is a necessary evil for sealed-headphones. The rotating earcups are a bit annoying when taking the M50 on and off, since sometimes you have to rotate the earcups back into the correct orientation. For traveling, the M50 folds down to about half of its normal size, and that makes it very easy to travel with, not to mention more durable because it's harder to accidentally bend them or twist them out of shape. The overall look of the M50 has a pleasant, no-none-sense professional appeal--they really do look like they were designed to feel right at home in professional studios.
On a side note, the Sennheiser HD280 Pro is often recommended to musicians who need sealed-cans, and I highly suggest anyone considering a pair of nice sealed cans check out the M50, as they walk all over the HD280 in every single way possible, while still remaining very reasonably priced.