Easy to use, contains music of new and older bands, quite cheap if you buy the entire album. Cons: No Beatles. Some countries don't have access to all of the music.
Able to complete an album if you've purchased a single track, without having to pay for the whole album (just the difference). Easy to use interface via iTunes. Cons: Too easy to purchase a Brittany Spears album when drunk :)
It syncs with iPods and iPhones. It has good OS integration on the Mac side. Cons: Some music previously purchased from the iTunes Store will only play in iTunes.