I liked it, just didn't like the interface. I was an early adopter and very excited about the protocol. When I was looking at it, there wasn't enough client specification to bother working on an alternative client (ie: emacs). Hopefully that has changed (or will change before Google drops support).

I found it really useful for collaborating with multiple parties. It allowed us to work on a document and keep the conversation all in one place yet clearly separated. Sometimes email can make that distinction difficult or unclear.

At least it's not entirely closed source. :)