I have now owned the M50 for slightly less than a week. They are more than OK for rock and other guitar music. They are great for electronic music (Aphex Twin, Boards of Canada etc.), superb for jazz and they shine and are doubleplusawesome on ambient. They are able to reproduce every little detail, both low and high. Every crackle crackles, every subtle percussion instrument sounds like it's right there next to you.
Soundstage is OK, but not special. My ATH-FC700 has a bigger soundstage. Build quality is excellent.
The comfort also is exceptional; I can wear them for hours and hours without a problem...although I haven't had them on while it was very hot yet.
In my conclusion, at this price point, you can get 'funner' (more bass slam etc.) phones, but you will lose in detail. Or you can with the M50's detail, but live with its more analytical sound. Very much recommended headphone!