What is Critical Mass?

Wikipedia has the best write up about Critical Mass in SF. Anyone can participate, and I'm pretty sure it occurs on the last Friday of each month. I have never rode in the event, and also am wondering if it's worth it?

Good info here

Below is a CM event in SF in 2005; do you know what street that is?

Critical Mass is a bicycling event typically held on the last Friday of every month in over 300 cities around the world. While the ride was originally founded in 1992 in San Francisco with the idea of drawing attention to how unfriendly the city was to cyclists, the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city or town streets on bikes.