It's a kind of mashup of email, instant messaging, social network, and collaborative work environment. Operating within a standards-compliant browser, and with a pretty straightforward interface (I write as a longtime Mac user who's more than usually sensitive to UI stuff).
Seems to work fairly reliably even now, though some bits aren't finished.
Think it looks like a Good Thing--a kind of Web 2.0 one-stop shop. Open source--there are already widgets to do things like automatically publish documents to blogs or wikis. I'm not a geek but I'm quite excited by Wave...
All right, maybe I am a geek...