Anyone else a member? Upon a friend's exhortation I signed up the other day. Only the other day though so I don't yet know how worthwhile it is or if it's just another dastardly privacy-invading internet phenomenon. :)
What it is: you create a profile, it then records all the songs you play in iTunes, and on your iPod when you sync it up with your iTunes. So after a while it becomes obvious what sorts of music you're into;
So then similar artists are recommended to you by the site; your friends can see what you've been listening to; and you're given a list of 'neighbours' - people with similar taste as yourself.
Furthermore, there's a little wikipedia-esque profile on practically all artists, and it lists how many plays they've had overall and the per-song breakdown - so you can see the most popular song when you're checking out a new artist, like say for example, that DJ Shadow's most listened to song is Organ Donor. Also, most songs have little 30-second previews. Finally, there's an event calendar thing where you can check all the upcoming gigs in your area, and like MySpace it lists the upcoming tour-dates on artists' profiles.
As I say, I'm a newcomer so can't fully judge, but I'd say if you've a good few friends on it (or maybe a foot group is possible, not sure) it might be decent. Anyone a fan already who can say more?