I really, really like
http://www.rdio.com/
Happy to throw them some money every month to be able to stream (or download to your mobile device) about 90% of the music you can think of (without ads).
Quality seems good enough to my non-audiophile ears. Biggest complaint, and it is a big one, is that there is occasionally a half/quarter second silence between tracks on an album. Since I mostly listen to albums straight, that can be really bothersome when the tracks blend together.