It's great fun to see what you're listening to and find others who share the same musical taste. I find the best part of Last.fm is discovering new music either on my own or having friends suggest new bands/CD's. If you like music, give it a whirl.
Interesting way to discover new artists and/or new songs based on your musical taste. Their scrobbler is well made and useful. I'd recommend, for only 3,00 EUR/month, to subscribe to the site as I'm really satisfied with it.