Critical Mass is not an organized group, rather "happenings" that happen to occur in many cities around the world on a regular basis. Critical Mass bike rides bring awareness to the availability of non-polluting alternatives to cars. They also make pretty obvious the lack of consideration city planners give to any form of getting around that doesn't involve cars. In the 'burbs it's worse yet (see pedestrian).
Tousles with cops are standard fare at these rides, since some traffic snarling tends to happen when large groups of cyclists ride together. (Especially when they're in costume or naked.) There's been lots of attention in NY to the cop who tackled a rider last week. But anyone who has ever attended a ride knows arrests, permanent confiscation of bicycles, and cops roughing up riders are all too common. Hopefully this media moment will get more bicycle activists involved.
All that, and the rides are still joyous events.