If you don't you really should check it out. In my opinion I think it makes talking to your friends while your playing a PC game much easier since it doesn't interupt your game. Also, it shows what games your friends are playing and makes it easier to join them in a game.
If anyone wants to, feel free to add me to their friends list on Xfire. My name on there is ac1dl3urn0216. Hopefully I'll get the chance to talk to some of you sometime soon. :)