I've been using these headphones as my main headphones for few years now, and while leather on the earpads and headband is showing wear and tear (I haven't oiled it up, which I probably should've done) the headphones have aged up very well.
Changeable cables is a must-have if you are going to use the same headphones on many different devices. The coiled cable allows me to move about in my room without taking the headphones off and the shorter cables are better for mobile usage. These are studio monitors, though, so they are rather big and don't sit that well around my neck. That's not something I would do, but I know many who do. Although the M50 X is mostly made of plastic, it's very sturdy and won't break in normal use.
When it comes to comfort, I've had better headphones. My ears are rather sensitive and the pads are not quite deep enough to keep my ears away from the headphones. Having said that, I sometimes have the headphones on my head for like 8 hours with only short breaks.
Talking about the earpads, the seal they form is very tight. This makes the M50 X, as I'm sure everyone agrees, bass heavy. The tone is warm and great for bass music and what not. I don't find the sound very tight, which with increased bass makes for a rather non-organic sound. The mids are clear and to my mind the highs sound good and are appropriately loud. They are not fatiguing in any way.
The biggest fallback to the sound has to be the sound stage, probably because of the tight seal. I've had IEMs with better stereo imaging. Velour earpads might change this, and for a closed over-ear headphone you probably shouldn't be expecting for the best lively, wide experience. Whats more, outside noise gets isolated really well.
While as it is I'm looking for a more neutral sound, the Audio Technica ATH-M50 X makes everything I listen to sound massive and pleasing at the same time. There might be better headphones for classical music, jazz or other very organic music, for any bass head EDM lover, rapper, or quitar player who likes a warm tone to name a few, ATH-M50 is the right choice: well built and cheap for it's sound.