Well, to get GoG wave on google wave really going, I have 5 invites I'm willing to give out.
In my opinion, if wave gets it's own stand-alone client and everything will go well, it'll be the service that would be very well able to surpass e-mail, IM services and many sharing services, since it combines them in a very accessible manner. You can dynamically switch from what seemed to be an e-mail conversation a while ago into IM conversation, and then invite more people into it, so it becomes a small IRC with people talking about matter that was previously discussed in IM and mail, with an easy access to all previous communication they were not part of. Of course, sharing of documents and pictures is easy and fast as well. And then, when you finish talking, you just throw them out and can communicate in privacy again. If you're interested into details, just look at google wave presentation video - yes, it's long.
It's an invite-only preview version, so expect some lag and it's not finished yet, but it looks very usable and practical. I'd like to try it to it's fullest, but one would need more people to do that now wouldn't he?