Pros: comfortable, blocks external sound
Cons: needs a battery, no good sound at all, very expensive for this ugly sound quality.
The bottom line: listen to other closed headphones (passive sound blocking) before being tempted by the good idea of active external sound blocking.
first i must say i don't own this headphone but i compared it to my $84 sennheiser hd280pro which is a closed headphone that also blocks external sound but not electronically and i must say the quietcomfort2 block better external sounds and is much more comfortable but sound is ugly: it is all muddy without bass or high frequencies, don'y buy this headphone before listening to it. the sennheiser hd280pro has much more details in high frequencies and a terrific bass. it is day and night - i don't understand people who like the sound of the quietcomfort2. it is obvious after listening just a few seconds to any type of musical instruments.

fyi : i also listened to the competitor from sony that also blocks the external sound electronically and it has the same problem : comfortable and good sound blocking but sound is horrible : bassy and muddy no details no high frequencies when compared to the hd280pro.

also be informed that if you are not needing external sound blocking, the best natural sound comes from my grado sr80 that cost $95 (ok the design and plastic quality of the sr80 are really cheap but the sound is amazingling exact and natural, the best sound i ever had from all my headphones in the $100 price category : sony,sennheiser,audio technica, grado).