Mainly I use it because I have an iPod and an iPhone. I hear that some native players have pretty good iPod support, so I may be changing over eventually. I don't really mind if it doesn't have iPhone support, because my iPhone doesn't have enough room for my music library anymore anyway. My main problem is if I was to change music applications, I would want the new one to be able to import meta-data from iTunes (play counts, ratings, playlists, etc.). It would also be nice if I could find a real cross-platform app, but that is a secondary concern.
