It's not cheap, but I pay for Spotify because overall I've been pretty happy with it. It has a good selection of features, lots of content, and it works pretty well for the most part. The only real complaint that I have is that the offline mode isn't great.
An actually cheap service that I pay for is addy.io, and even though I haven't been using it much, so far it has been working pretty well.
Same thing here. I can't seem to figure out how it decides what to play. Sometimes I'll actually forget that I added certain songs to my playlist because one day Spotify will just decide to play it.