First I should point out that these types of headphones do have issues in terms of reliability, where one ear piece will eventually give way. This is a problem with many in-ear headphone products, I have not had a pair that do not have this issue (that has been many).
These Skullcandy products do tend to go a bit sooner than usual, however Skullcandy provide a 100% lifetime guarantee on all headphone products (check website), it couldn't be simpler, pop them in an envelope with a brief description of the issue and the original receipt, within no time you will have a brand new pair.
Aside from this issue I have personally found them to offer superb comfort and sound quality.
Before I get lambasted about the quality of sound produced I would like to point out one thing, these are going to be used on portable MP3 players, as such the quality of the file comes into play, if you are seriously into music you won't be listening to .MP3 files.
In comparison to other in-ear headphones I find them to be head and shoulders above the competition. They have a snug comfortable fit within 'my' ears. The quality of sound across the whole spectrum is excellent. Also, ignore expensive noise cancelling headphones, with these in your ears you really aren't going to hear much else.
All in all, for sub 20GBP, listening on an MP3 player, you aren't going to get much better.