Hopefully it will gain more usage now that it's not directly tied to Google's services. It had potential for collaborative engineering and role-playing games.
I used Wave while working on setting up an organized gaming event. Communicating with several people and organizing data into groups was simple and effective. Not sure what we would have used otherwise. I'm just glad it will be around when we need it again.
Google Wave was a great idea and was well implemented, but like many others suffers from the paradox of not being very useful until a lot of people are using it.