I'll start by saying that these are excellent for travel on an aeroplane and for studying/office work. I bought these early into a five week holiday to Japan/Australia/Thailand as in total I was going to be flying for over 60 hours (I actually got them from the Sony centre in Tokyo - just thought I'd throw that in there!). My first reason for only four stars is that although the noise cancelling is strong - I am using them now and cannot hear myself typing - it wasn't entirely enough to block out all the engine noise. That said I could clearly hear the in flight 'entertainment' without the volume too high. They also worked well amplifying my iPod Nano to an audible level on a plane - the Nano is not the most powerful of devices. They spent five weeks in and out of my rucksack and I have not experienced any issues with wear and tear - an added benefit is that they fold to a compact size.
One caveat I should mention is that the noise cancelling was unusable on some of the pot hole covered roads I experienced in Thailand as the frequent violent shaking caused skipping with what was playing. I thought this may have been due to my music player but I confirmed it was the headphones by switching off the noise cancellation - at which point the sound was uninterrupted. This problem did not occur during turbulence when flying. I would say that these headphones are mainly for normal travel i.e. planes, trains, the underground, normal road conditions - my version even came with a two plug aeroplane adapter. They have since served me well back in cold and wet England as a method to drown out my flatmates whilst finishing last minute coursework.
For anything other than extreme sports I recommend them highly, especially for the relatively low price.