I used Wave quite a lot, it was great to arranging meet ups of groups of people as you could all have an active conversation and use the "Poll" widget to say if you where coming or not.
I also used it for other things, but organising groups of people was the main use. Once it was discontinued I tried to run the open source version, but it was never really that stable and in the end we swapped back to emails.
Yes, it was really great for organizing groups of people. My campus ACM chapter used it instead of email when we were setting up a new computer lab, and it was one of the best experiences I've had with a piece of collaboration software.
It's main problem for us was that large waves tended to get slow on our 2007-era entry level laptops, and nobody really wanted to maintain the wave by deleting unimportant chit-chat. Some sort of decay mechanism to hide old, irrelevant blips would have been nice.
I also used it for other things, but organising groups of people was the main use. Once it was discontinued I tried to run the open source version, but it was never really that stable and in the end we swapped back to emails.
It's a great shame to see this dead