I kind of like them for the sheer novelty of it, infrequently, but if I tried to do an organized mass ride every month I'd avoid the big charities like the plague. Even the charity ride that I do spends much more on marketing and promotion than it donates, so I don't really get the warm fuzzies from fund-raising. I'll just plump the minimum down if I do it again.
There's a local century that ends up running through my regular cycling routes, and since I'm out quite a bit during those times a group will often catch up with me or vice versa. I'd be offended if someone suggested that I should clear off the road for their ride, but that's the last thing on a rider's mind deep into a ride. The only thing I feel a little guilty about is going up that last hill (on my way home) still relatively fresh near the riders with 80 or 100 miles already. I wouldn't drive out and jump in somewhere along their route and follow along though; that would feel classless.