I don’t know that it “replaces” e-mail so much that it does a lot of things far better than e-mail. Nor does it “replace” instant messaging, but a conversation in Wave totally blows any instant messaging system out of the water.
Trying to manage a complex project via e-mail or IM is a recipe for frustration and confusion. Wave combines the immediacy of IM, the permanency of e-mail and the flexibility of a blog.
Plenty of people are still very excited about this. If you have a Wave account, go to Extensions in the Navigation pane and have a look at all the things Wave fans have created. Basically, right now it’s at the Netscape 0.97 stage compared to what it will be like.