i started using amazon when itunes had a problem downloading MP3s properly - i keep my mp3 library on a NAS drive and amazon would write the file to C: before moving to the network, never seemed to have a problem. Itunes had a problem with this for a year or so before it seemed to resolve. itunes support was not helpful with this.
Plus - amazon has no conversion issues. i sometimes speed up songs for running using audacity, and you need to convert mp4s to mp3s before you can do this. with amazon - no conversion required.
not to mention that it is cheaper, sometimes 5.49 vs 9.99 for an album.
and now you can use the amazon cloudplayer from any computer. and they were giving away a special on cloud storage so i uploaded a copy of my old MP3s to amazon.