So I got a Google Wave invite yesterday morning and immediately signed up. The interface is nice, if a bit slow at times on my 2.16 GHz MacBook Pro with 2 GB of RAM. It also ate 1 GB at one point and Firefox started acting weirdly (left clicks on flash apps were generating right click behavior). Firefox normally eats around 100 MB of RAM on my machine, but right now with Google Wave open, and having only worked with two waves, it is eating 300 MB.
If have mostly used it like email so far, I haven't had a chance to play with the IM like features. I have used the wiki-like editing feature though, replayed the history of a couple of waves, and used the map gadget and everything worked pretty much the way I expected it to. I played a little with the kasyntaxy bot and it was interesting, but its Perl syntax highlighting leaves a lot to be desired.
If you are on Google Wave as well, drop me a line at chas.owens.