We used it on my team at work and it was fantastic for design discussions and the like. Incidentally, while slack, dropbox paper etc all target this space somewhat successfully, I think there's still a lot that could be done here.

Getting a huge user base for wave would have been easy - simply incorporate its features gradually into gmail, and make it so that if you're in a threaded conversation with someone with a wave server you have all the extra features, otherwise it's just normal email.

This is in fact what I assumed google were up to, since they were huge on the email front and they described wave as the future of email.