It has some interesting features. I like that one can instantaneously switch between chatting live and asynchronous messaging. I also like the gadgets, I can write equations in wave-messages using latex, something which is always a pain when writing e-mail.
I've got more invites than I need at the moment. Send me a PM if you want one, although one really has to have a few friends or coworkes on the wave to explore its usefulness.