I managed to get my hands on a Google Wave account yesterday. It's tons of fun - but waaaaaaaaaaaaaaaaaaaay too overwhelming. I don't think I'm allowed to reveal any screenshots as such, but what I can say is that it's still buggy.
So how would I put Google Wave as? Let's say Facebook + IM + Email + Wikis + Collaborations + widgets/apps... not sure, but I think it pretty much redefines "social".
So far, it's slow as well, but that's probably because it's using HTTPS.