[a-french-anon]

Reading your comment and all its subtree made me realize there's another difficulty to the problem: what atomic unit do you use? Tracks, albums, artists?

One might argue that "artist" isn't granular enough, since lots of (most?) artists change sound during their career. For the two others, I think recommendations should be trained and given separately (segregated, if you will) between people who listen to albums and those who only care about tracks/singles.