I took part in a critical mass here in Québec. Think of a clump of cyclists ignoring red lights and such, clogging up the roads and taking pleasure in holding up traffic... Without saying this is bad for cyclist - motorist relations on our roads, I've yet to see any shred of evidence at all that CM's have done good for cyclists in cities that have them. To me, they seem more like a monthly event for semi-anarchists... I never did take part in another CM ride after that first one.
I prefer to do my cyclist advocacy on a daily basis, by showing the example. The only way to be treated as though you belong on the roads is to behave as such. This is why I use hand signals, stop at red lights and otherwise complying with traffic control installations, using proper lane position, etc. (at least if there's other traffic around, otherwise...) Motorists occasionally do show signs of appreciating these courtesies.
http://en.wikipedia.org/wiki/Vehicular_cycling
http://www.bikexprt.com/streetsmarts/usa/index.htm