Having played around with Wave, I'm beginning to see some of the uses. Google describes it as a tool for collaborative document editing, but to really grasp it's usefulness, you need to have a fairly loose definition for what constitutes a "document".
A document could be a proposed set list, or a set of suggestions for where to eat lunch, or a meeting agenda, or anything like that. I've been using it mostly as a notepad for things that I might want to share, particularly with bandmates. It's the kind of thing I use Google docs for, actually, but it's more flexible in some ways, especially with respect to sharing information.
Of course, the big drawback is still that no one actually checks their waves, but that will change as the usefulness becomes more evident. I'm still not convinced that it's any kind of be-all-end-all toy, but I'm a lot more optimistic than I was a few months ago.
Incidentally, I've got like a dozen invites, so if you or anyone you know wants to play around in it, just ask.