Spotify premium is a musical journey. The sound quality is very good, and the choice of music is incredible, with features like "similar artists" you will never look back. I have discovered about 10 new artists this weekend alone. You can share playlists (just playing XFM's 960 song playlist), and download songs to play offline. I have the spotify app on my htc desire and it has replaced the ipod.
Back to the sound quality, I am streaming through my squeezebox and the sound quality is very good (IMO). Not sure if I will buy any music again, give it a go, you wont regret it.
It's all about the music after all.