I'm a paid subscriber and I'd like to correct what I think may be a faulty assumption (which may be due to poor wording in the original article).
Spotify does more than let you listen to the radio - they have a large catalogue of music that you can search through and play at whim.
You can also save music to your mobile device and listen to it later. If you want, you can go to a friend's house, login to Spotify on your account and then listen to your music there.
http://www.spotify.com/uk/about/features/
All this for just £10 a month! For all intents and purposes, Spotify is "all-you-can-eat" for the needs of most people. I wish they'd improve the range of tracks they have but that's somewhat beyond their control - this will always happen until the market tips enough to get certain old-world businesses on board.
Before this comment runs the risk of sounding like a hagiography, I should point out that the features for creating and sharing playlists are fairly limited and the client software is fairly basic (at least on OSX, which is the only version I've used).
Edit: Beaten to the punch by ladon86 by a couple of minutes. I should learn to type faster, or perhaps just use fewer words :-)