I wanted to join this list, so I DL'ed XFire. Installed it, signed up, etc. and after going to play Crysis for a while and then quitting, the computer BSOD'ed, not once, but twice. Message was some sort of corrupt list. After uninstalling XFire and clearing the registry of any marks of it, and then restarting my PC and playing Crysis again, it didn't happen.
So, pass on XFire.