After around 100 hours (probably more, since I left it on my computer once while the music was playing and forgot all about it) on my ATH-M50, I think they're really worth the price.
The sounds that you get are really quite clear and sharp. I listen to mostly korean and japanese music (mostly upbeat, bassy dance songs, and some bands) and it performs quite well. What I really like about the M50s is the fact that I am now able to hear some sounds that I have not heard before in my music files. I mean I can hear some distinct cheers during concerts or some instruments that I previously did not notice when using some lower type of head or earphones.
Of course, there is a problem to it too. Some poor rips (i had 112kbps rips of some music back when I was saving hard drive space) are bad as you can easily hear some crackle or noise.
Another thing, Sound doesn't seem to leak too. I used them on the plane, and the people seating next to me didn't seem to mind. They said they didn't really hear a thing when I had them on. I don't think they provide very good isolation you though, you can still hear some chatter or some background noise.
A couple of days ago, I had my ATH-SJ3 fixed (they were my first headphones, got them for design). I then listened to my music collection on them, and I was amused. I remembered when I first got my SJ3s, I thought they were a very big upgrade over my stock earbuds/sennheiser earbuds (mx something, the cheapest ones).
But after spending some time with my M50, I already began to realize just how muddled the sound quality of the SJ3 was. I am (my wallet is) hoping that this would not lead to a scary slope wherein I would begin purchasing better and more expensive headphones :D I think the M50s are a gateway drug to the world of audiophiles.
I am using it now (just bought it today) with a Fiio E1 amp. My source is mainly an ipod touch, and i truly enjoyed the ability to control it using the remote. I auditioned it with an E1 and an E5 and honestly, the e5 sounded a tad better (more bass) but I really liked the remote functionality so I got that.
However, I don't think you really need to spend on an amp for the M50 as it can sound quite nice unamped anyway.
Cable are tough, tips are durable, but they seem to be too long for portable purposes (which I usually do)add the 2 feet of the Fiio E1 and I really have a whip with me when I go around.
I get around this by using velcro strips for the headphone cable and tie wire for the Fiio E1 :D