The genres/styles of music is rather limited, but this has been great in terms of generating music background for my short YouTube videos (max length of song is 5 minutes). You can specify exactly how long you want your song to be, after the initial sample is created.

The 50-download-a-day plan is pretty generous. It seems like you can generate as much as you want, but you're only deducted credits if you like the result and download it. I'm enjoying this!