After bunch of comparisons I decided to get a Spotify Premium membership about a week ago. So far I like it. I guess this means I can finally delete all the questionable music I've acquired. The library is pretty good though they don't have a lot of Red Hot Chili Peppers or other major bands but I like how it integrates with Facebook and my Android phone for offline listening of playlists. Also the music has a 320 kbps so it's really high quality.
There are a lot of tiny things that I've noticed. In Win7, you can see how far into a track you are by looking at the icon in the taskbar.I also have it scrobbling to Last.fm. No sense getting rid of 5 years of listening data.