I spent a solid 12 hours hosting the Debian Party Line chat today thru the Debian Squeeze release process. That's much much more time than I usually spend on the phone or talking.
It went well. The mumble server peaked at 55 people connected, and 200 unique addresses connected to it over the course of the day. We had some good discussions, and a lot of lurkers too. And not too much bother with badly set up microphones.
I hope that Debian will find uses for mumble going forward, some ideas I've heard are to use it for team meetings, or for remote audience feedback at DebConf talks. I plan to leave the server up as long as it is not abused.