Joined this myself couple of days ago. It's alright. I'm skeptical they'll be able to overtake Facebook, but then again, if anyone can do it, it's Google with all the other programs they have to offer along with +.
I like it more than Facebook already. Facebook has too much personal things, and too much stalker stuff. Still won't be twitter yet. Add me to your circle: Jonathan De La Roca
I haven't been back since the first time I used it. The interface is not very user friendly. Btw the iPhone app is already out. Now we need a WP7 app for me to start using it. But I might switch to the new iPhone.