I was tired of having to manage different playlists on my phone and desktop; I wanted to be able to play my songs from anywhere, in a seamless, continuous experience. The cloud-based service makes this easy, as all that's needed is an internet connection to automatically sync my music and playback.
The target user is anyone who wants their music to be available everywhere, and to continue listening from exactly where they left off. If you think it would be cool to be listening to your music on your computer, pause your music, then leave to go for a walk and have your phone continue playing from where you left off, then this service is for you!
I am most proud of the frontend web music player; it looks good and works well. The resume-from-where-you-left-off feature works surprisingly well, as do bulk uploads of music files to S3.