Yipee! I just got my Google Wave account and activated it. I have been playing around with it when I have a few minutes. My initial impression is that it's going to take me a bit to get the feel of what I can do with a "wave."
I have seen web-based collaboration software before with some of the features (like real time messaging) that I see in Wave. However, Wave adds nifty new features like a playback mode where you can step through each individual contribution to the Wave (someone typed in, posted a video whatever) to get a sense of the history of what happened.
Wave is a work in progress -- when I tried to do playback and then fast forward to the last message of a large wave, Wave crashed. It recovered gracefully though, and did not crash my browser -- it just told me I needed to refresh the page.
I'll try to write up some more impressions when I get more familiar with the tool.