I recently tried my first group ride with a single-speed bike (not a fixie, tho). Worked okay for the most part, but towards the end of it, I found my legs tired, and I was going uphill into the wind- sure would have been handy to downshift and I couldn't. Moral is that for this to work well, you better have your legs in great shape. And if they're in that great of shape, a single speed is going to slow you down on all the easy spots.