I got an invitation to Wave and tested it out earlier. There is a lot to like but it's clear that the reason they've restricted the invites is because the whole thing is seriously buggy. There's a lot of kinks for them to get out before opening it out.
I love the idea and think that there is a lot to improve on email, IM and current collaboration tools. This won't work though unless it was an open standard and everyone could build a client or host a server. I can't see Google allowing that. It's a pity because I do really like what they've done.