Just now I was reading "Divorce Your Car!", and on pp. 160-1 this very topic is mentioned. Some critical mass rides run lights to keep the group together. It is alleged by some that the rides have helped gain local government support for cycling.
Anyway, my opinion is that a more effective model than the one I've seen in youtube videos would be to have a longer string of bicycles, scrupulously obeying the traffic rules. Sticking together in one mass would be less important than filling a number of blocks for a period of time--filling it well so as to do nothing that could be criticized, except existing of course.
In some videos the cyclists actually head into traffic going the wrong way. Ridiculous!
Many blocks of competent cyclists would be a good and competent impression. A brief chunk of cyclists behaving oddly is merely an event to be questioned.