The “critical mass” movement receives mixed responses. I suppose motorists are probably pretty consistent in their reactions, but the opinions of cyclists vary. I’ve never been part of one, but I’ve thought it would be fun to do so, and I’m of the persuasion that they have the potential to be quite productive (notwithstanding their potential to be equally divisive as well).
Here’s a piece on Detroit’s critical mass scene from the Huffington Post. If you have any interest on the history of the movement, check out One Less Car: Bicycling and the Politics of Automobility, by Zack Furness.