I don't know whether you know, but Spotify also has a basic package, which is just streaming for free with some advertising in every 20 minutes or so. It's just like the good old invention, called commercial radio, but with the songs you want.
Also, what makes Spotify an excellent choice for music is that you can purchase songs and you can listen to the ones on your computer without opening iTunes. Since I started using Spotify, I only used iTunes once - for downgrading the OS on my iPhone. Even that was something that Apple doesn't approve of so I had to 'hack' a bit.
But of course, if you like, you can pay the premium fee and use the service without interruptions, even on your mobile or offline.