The sound from these cans is superb and detailed. The quality is as good as my now rather ancient HD 595s, which I've used as my "reference" phones for some years, however, the bass is quite a lot more prominent. If you like rock music, these are a good choice for you! I've used a variety of sources, including an iPod, a Zune, computer, amplifier and in-flight entertainment systems and had good sound with plenty of volume from all these sources.
In terms of noise isolation, I agree with the other reviewers, these attenuate ambient noise but you can still hear clearly what's going on around. On a long-haul flight the background roar and announcements were still audible (if I didn't have the volume cranked up to painful...), on the London Underground I could hear announcements, and in the office I could hear the phone next to me ring, but in all environments, I could still enjoy the music or follow the film dialogue easily.
So why not five stars? The main reason is the curly lead which is a nuisance, especially when using it on the train or plane, and even sometimes at home. I'd much prefer a straight lead.