It isn't that bad, I would say I'm listening to pretty broad spectrum of music, most of it being not very popular, and Spotify has roughly 75% of what I need. OK, when something that was there disappears after a while then it's seriously irritating, but still, the true selling point of Spotify is the quality of the program itself: it's super slick, fast and has the best interface ever in the category of music players.

Personally, I ignored a bunch of artist just because they don't have their music in Spotify; "you want me to listen to your music? let me do that the way I want, otherwise go screw yourself" type of thing.