Fourth, I've never found a recommendation engine I like. Maybe I have weird taste in music, but Spotify only ever recommended one song that I ended up loving (oddly, the very first one it ever did).
Sure, I've had some good recommendations, but I'd expect any recommendation engine to do that just by chance.
What I can say is that Spotify has never proposed something truly new and interesting to me. If I listen to a lot of metal, it'll recommend me the most bland stuff in the same genre. What's even worse is that it keeps playing the same songs over and over again. It's like the recommendation engine just gives up and starts repeating its suggestions again.
I simply gave up on it. Why have access to everything when I only got the crap. Going back to my personal collection increased my good/bad song ratio considerably.