The sound quality of these headphones is excellent. However, the build quality of these is absolutely terrible. I received a pair for a present about three years ago and I have literally been through four or five pairs since. I cannot fault Bose, who offer excellent customer service and have replaced each pair for free.
The main problem seems to be the connection between the jack and the wire, as every pair I have had has started to cut out in the left ear after about ten months, progressing to the sound in that ear cutting out completely). Also, in earlier pairs I had the ear buds kept popping off and although Bose have refined the design recently, the buds still come off occasionally (which is infuriating... especially if you have to scurry around on the floor looking for them).
It is a shame as I really like the sound quality, comfort and level of service that Bose offer. However my fifth pair (changed at the Bose shop December 2008) have now started to cut out in the left ear (it is now October 2009) and bearing in mind that I do not subject them to adverse wear and tear, I find this pretty unnacceptable. I would recommend these if you only ever sit down and listen to music (as my father does), otherwise I would consider something else on the market.