We had a prototype meet-up last night. 8 people at the most-full (limit is 10 per hangout.)
Having mics up seemed to work well before the show and during the musical segment. Even though there was some funny back & forth during the first part of the show proper on the mics, the consensus formed pretty quickly that we all preferred to mute mics and just go text and video during the actual show.
It was fun, but oddly exhausting for me. And it felt a little odd at times just watching people listening to a radio show.
Final analysis: I enjoyed getting together with FoTs on G+, but maybe getting together to listen to the show isn't the best way to use the feature.