I'm in, but can't do any invites. I'll touch base here when they open it up again and you can actually register with the invite again.
So far it's pretty good. The website is good, what's happening with your information is much easier to understand and control than facebook. So far it looks like it asks you for permission every time you run things for the first time that will share your information.
It integrates with Picassa and that seems to work pretty well. The phone app also works pretty well. However, on the phone app there are some issues with screen space. It looks pretty good when you have your phone oriented up and down (portrait) but when you turn it sideways (landscape) stuff like the Google+ logo or the bar that shows the pictures of people that you're talking to take up a huge portion of your screen. You end up trying to see the posts, pictures or chat stuff in a portion of your screen less than 1/2 the height of your screen. I have a slider phone so I'm going to be using it sideways a good amount of the time and this does bug me.
Huddle works very well, it's like a mobile chat room.