I've never written a review before but feel compelled to now because I bought these headphones last week and thought the sound was incredible. Yes, there is a slight hiss from the noise cancellation but it is barely noticeable and you get used to it very quickly. I heard things in songs I never knew were there, beautiful.
I had to return them the very next day because they leak SO MUCH SOUND. It's not like just hearing someone has music on, it is literally hearing every lyric. Heard from other online reviews that people have been asked to remove them countless times. As I bought these for out and about and for travel, I had to return them as they would be totally unsuable on a bus, train or plane. I strongly suggest you think carefully about where you want to use these before you buy them as with the volume on a nice level you might as well be playing music our your phone. Even on a low level you can still hear the lyrics.
So decide wisely. If you're a DJ or something I would imagine they are marvellous but if there is ever going to be anyone around when you use these headphones, I suggest you think carefully.