Yes, I'm a bit of a cheerleader for the project, but seriously: it's worthwhile to sign up and get a feel for it. When it first came out, it was basically an early alpha: slow, buggy and missing a bunch of necessary features. It now deserves the beta label -- it's a good ways from done yet, but it's genuinely useful and a lot more stable. It's still hellishly demanding on both your bandwidth and browser (translation: you still can't use it with Internet Explorer, which is just too slow), but seems to work pretty well and is gradually getting easier to use.
The standard question is: Why would I want to use it? The answer is for collaborating on documents. At the moment, the sweet spot is still working together on text that is content-heavy and format-light: if you have a bunch of people who are trying to collaborate on a text project, Wave is the best tool I'm aware of, combining the best elements of collaborative editing, mind-mapping and conversation. It's particularly excellent for meaty projects where you want to be able to work together in real-time when possible, but where not everybody can necessarily be there at the same time.
There are a bunch of other good applications as well. Brainstorming seems to be another sweet spot: several of the most useful Waves I've participated in have been multi-day brainstorming sessions. And people are slowly hooking in gadgets to make it useful in more ways. But if for nothing else, I've found it enormously useful for cases where we have 3-20 people who need to produce a document over the span of a few days. It's not the only tool for such jobs, but it does seem to be the best one to date.
So I recommend checking it out. Feel free to poke at me there (my usual handle @googlewave.com) -- it's also good for random chat, and conversations in Wave are one of the better ways to learn it. The access-control tools are still a bit weak, and it doesn't have the social networking capabilities I so desperately crave, but I expect it to eventually become a kick-ass general conversation system, and maybe eventually a genuine rival to LJ...
