I just signed up today myself, so I don't know how well it works. But I am absolutely in love with the concept. I subscribe to band mailing lists and RSS feeds and iLike pages and on and on, yet I still seem to miss new albums on occasion.
Honestly, I just wish iTunes would do this for me. It knows what songs I have. It can figure out how often I play them, what I rate them, what fraction of the catalog I own, etc. It can know what are truly my favorite bands.