Its the opposite for me. I now rely on Spotify for 95% of my music and have only bought some CDs for listening to in the car and the occasional band who are not on their catalogue, yet. With downloads at low bit rates and importing CDs onto the PC there was a justification to have the CD as a quality backup. But now, with Spotify you can download albums onto the PC and make up playlists and listen to them off line. So Spotify itself is the back-up. (Thank goodness or else I would be bankrupt, buying numerous, expensive, rare Swedish prog rock albums off Amazon at £20 plus a pop).
Best of all in my system Spotify sounds amazing, with loads of spine tingling moments. Rhythm, timing, dynamics, sound stage, detail, separation, bass, treble, midrange are all there in spades. I have got some sort of synergy going on that makes it perfect for me.
The one thing I would love to be able to do is to borrow a really good CDP and plug it into my amp and listen to some CDs again. That would be a true comparison with Spotify. I doubt that it would be significantly better as neither itunes nor WMP nor LastFM are at all better than the sound from Spotify. Even if it turned out to be a simply amazing sound, I would not give up on Spotify due to its cheapness for me, as I dont buy the CDs and the sheer size of its catalogue.