I've never been a bike messenger nor have I played one on the Internet. I'm a SS/FG devotee, but I really don't understand why anyone who makes their living riding a bike in city traffic all day would want to do it on a brakeless fixed gear. If I were doing this daily, I would probably use a fixed gear too, but with both front and rear brakes. I would gear the bike on the low end. This would give max control of the bike (if you are skidding you ain't stopping), and not put so much wear and tear on your knees. I'm coming at this as a geezer who has been riding a bike for 61 years. I was off the bike from '92-'98 with palellar tendonitis. That weren't no fun, boys.