After I uploaded my music collection to google music I've since bought 50 or so songs on the service. Before that I'd only purchased one CD from a local band in the last decade. So a good solid product at a reasonable price converted me.
Are you using all access? I don't buy most music now, since it's on GMAA.. makes me feel guilty, but also happy heh
I used All Access for a while but if I really wanted some music, I often went and bought it. That way, even though I don't subscribe anymore, I still have some of the music I really like.