The only good one I've ever seen was in the music business: Rdio, specifically. (Now sadly dead.) I don't know what they were doing, but their recommendations were consistently excellent at pushing me towards new artists and even new genres that I didn't expect but would up enjoying.

Back when I used to use it (about a decade ago), Pandora was good at shuffling, but their library was so small I would get the same songs over and over.