At the recent SQLSaturday #4 we had a session end up finishing much earlier than expected, so we gave the attendees the option of moving to other sessions or staying for a Q&A session done with a panel up front. I've only done the panel format a couple times and by the time we finished realized we needed to learn a few lessons for next time. One is to decide if everyone should comment on each question. I think the ideal is whoever answers gives a complete/deep answer (to the extent it makes sense) and the others only join in if they have a contrary opinion. The complete answer is important, but hard to know - who can give the best answer out of the panel? We sort of went with let the most junior panel member try first, then work up from there, but I think it subtly makes the junior guy look junior - probably not fair. Perhaps another/better approach would be to make sure the panel members have similar skill levels.
I'd like to see a couple of these at each community event, there are a lot of questions waiting to be asked.