I know this group has been discussed a lot here, but this is my first experience with it...
The organizer posted an ad for a Critical Mass ride on Craigslist leaving from the U of Maryland campus at 5 pm. I'd never been so I thought I would try it out. Only 6 people showed up because of the rain. Everyone was very nice and friendly and they were mainly also students. 2 of us (including me) had blinkers and helmets, 1 had blinker but no helmet, and 2 had neither. Also, one of them had a singlespeed with only a coaster brake, and his chain was too long and droopy, which drove me nuts and made me want to whip out a frickin' chain tool
We only rode about 8 miles. It was fun but kind of scary to ride around in a pack on 40 mph roads. The leaders were good about signalling when we were turning and stopping at stop signs when there were cars waiting, but a couple people tended to swerve a bit. Our pace was reallly slow, like 10-12 mph on the flats, and uneven, so there were constantly cars getting stuck behind us. Although only a few actually honked, I imagine a lot more were pissed off.
As I said, it was fun, but I think it is just a little silly. I realize that bicycles have the same rights to the road, but it is inconsiderate and unsafe in my opinion to take up a whole lane with a bunch of very slow moving bicycles at rush hour. No one likes being stuck behind a slow-moving farmer on a tractor, but where I am from they are mostly polite enough to ride on the shoulder.
When I commute on my bicycle on my own, I try to use paved trails and less trafficked roads. If the shoulder is wide and smooth enough I use it. If there are no cars at a stop sign and I can see clearly, I'll coast through it. Instead of going through a left turn at a busy intersection, I'll often cut over onto the left-side sidewalk and go around the corner on it, then cut back across when the road is clear. I realize that some of what I do is technically illegal, and some of it is ceding my equal right to the road, but I feel like overall I am not doing anything unsafe and I am making the ride more pleasant for myself and car drivers.