Yes, it keeps your mp3 player dry. Hope they bring out a smaller one as I use a nano or a sansa clip player so the case is actually too big for me. Great to listen to music.
However, if you do want to listen to music in the pool (and swim crawl) I suggest you spend more and buy a Finis MP3 player. The issue being the headphones having a cable. It's amazing how the cable actually moves around in the water and can come out of your ears so earphoneless MP3 players are what you will probably end up going for. The issue with the Finis player is that it doesn't work as well out of the water so you will need one of these as well if you like to go canoeing or surfing and want to keep your music player dry.