Since I was riding a friend's Kona Unit (singlespeed), I think this forum is appropriate.
I'm not going to romanticize Critical Mass (CM). Essentially it's a group of cyclists taking over a street. From my experience, we pissed off a lot of drivers.
However, I have never felt so safe on the road as I did during CM. At one point I looked behind me and there was about a block and a half of cyclists. Incredible. I'm sure there have been previous CM's where there were more, but this was significant to me at least.
I have to say, it was nice to feel like a car.