It’s not a home run for me, but I think Spotify recommendations are quite good. They clearly use some form of content based recommendation (extracting features from the music itself) blended with other methods. It seems to make an honest attempt at serendipity (songs/artists you may like but would otherwise be unlikely to discover).

I still think recommender engines should always enable some form of user tuning. If it doesn’t, then the recommender is a tool for services to control your behavior rather than the other way around.