Sometime after we finished our D&D 3.5 campaign last year, Roll20 switched the engine it was using for WebRTC. Apparently this caused lots of problems for everyone, but I was hoping that these issues would be resolved by the time we started our new campaign.
It appears that I was being too optimistic, and we were unable to get more than three people working over Roll20's video chat (and the forum thread discussing the issues are still going strong). So we tried using Hangouts external to Roll20, but one person was unable to connect to this (not sure why, since Hangouts has always 'just worked' for me, on several different platforms).
In the end we tried appear.in, and this just worked without anyone needing to sign in and register. The only hiccup was one person with a delayed audio, which was rectified after a disconnect/reconnect.
So we've now disabled video/audio chat in Roll20, and are planning on using appear.in for future sessions. We still use Roll20 for text chat, and of course all the dice rolling, character sheets and mapping (at which it still excels), but the video chat seems to be unusable for us at the moment.