A great closed set of headphones.
First of all I should say that I love these. They're lightweight, innocuous, and do a good job with muffling sounds outside. I've taken these on a flight and they are quite lovely. Instead of that awful humming from active NC (think Bose QCs), they merely attenuate all sound around me. I can still hear the flight attendents, yet for some reason can't hear the baby three seats behind me.
As for sound quality, these are quite lovely. They have a nice, deep and accurate bass that helps distract from the din of the outside world. Vocals are presented quite well, and as everyone else has said here... treble could use some work. I've only noticed the annoyance in classical pieces, more specifically with strings.
These headphones are built well. I've already dropped them a couple of times, and they haven't had a scratch. Plus even if you do manage to break them outside of warranty or are at fault for breaking them Sennheiser has a parts shop to replace most any part.
In short this headphone is a great one. Perhaps the best I've ever encountered for a closed headphone, and my new favorite for portable use. Sure they're a bit uncomfortable, bad for classical strings, and look butt ugly... but that just means that they're great at isolation, good at just about every other kind of music, and have little appeal to a mugger.
