I think they used to market it as "we'll find you all sorts of cool music", so the expectation I had was that I'd be able to type in an artist or song and it would run an iTunes-esque "Genius" on it to give you a playlist.
Instead, I'd type in an artist or song and it would come back with no results.
It was happy to play music that I already had on my machine, but seemingly had no capability to go out and find new music from the internet. Given that that was its stated purpose, I was annoyed and uninstalled it.
It's probably worth giving it a go but unless you have very mainstream musical tastes, expect to be at least somewhat disappointed.
Their catalogue isn't too bad for a free service (probably around 40% of what I search for comes up with useful results) although having to listen to adverts every four songs seems a bit much, and it's certainly not worth paying to get rid of them.