I've finally signed up at last.fm. I don't know why I resisted for so long, but the increase in the numbers of full tracks that they feature was certainly an encouragement :-D. I do sometimes listen to the radio stations at work when I'm away from my main iTunes library, but I'm mainly interested in it as a way of discovering new artists. A 'similar artists' station turned up 'Iron & Wine', who I had never heard of before. I liked him (yes, it is just a 'him' rather than a 'they') so much that I bought 'The Shepherd's Dog' recently, as you can see from my recently scrobbled tracks.
It's also interesting to look back at your listening habits. It isn't completely characteristic of all my music listening, because I also listen to CDs on the stereo downstairs, but I seem to oscillate between fairly random playlists of a wide range of my collection and intensively listening to a few albums straight through. I'm in the latter mode right now, it seems.
I haven't really done anything with the social side of last.fm yet (if you're on there and think you might enjoy my musical tastes, do point me to your username), and I wish that the player integrated with iTunes rather than using a standalone player, but otherwise I'm liking it a lot.