A friend of mine recently sent over a Google Wave invite, so naturally I went and checked it out. 
Here’s my impressions of it so far…
From Wikipedia, Google Wave is a communication and collaboration tool designed to merge common networking applications like e-mail, instant messaging and more. It works in real-time, so people can see what others are contributing as they write.
Each “wave” is essentially a thread (analogous to forum topics and chained emails) where people can jump in anywhere to add content. Some examples of use can be found on a page published by Google. I won’t go too much into detail on that, since I haven’t had the chance to use it in such an environment.
What’s interesting about Wave (or at least the Wave protocol) is that most of the code will be released as open source. As such, developers can join in and add their own applications/extensions, making the whole works even more useful. In terms of functionality, it obviously surpasses e-mail (and I believe one of Google’s goals is to supplant it with the Wave protocol).
Will Wave be successful? Sure. I can definitely see people using it in collaborative environments (say, working on a large project at work ). It may even replace certain social networking applications given the right extensions. However, a goal like replacing e-mail and instant messaging… I dunno. What’s great about them is their simplicity; if I want to get a point across, I can quickly shoot off a block of text. In addition, pretty much everyone who uses the internet knows about e-mail; the same can’t be said of new gadgets like social networks and whatnot.
I suppose as the general populace becomes more technically adept, they’ll begin to see the benefits of using stuff like Google Wave. What will really make or break the protocol is how readily people are willing to accept it in lieu of traditional tools. Some sort of backward-compatibility with widely-used applications would surely go a long way in gaining acceptance.