i run a front brake, but i'm building another fixie from a track frame with no brake holes, so i won't run brakes on that one.

brakes are not necessary if you watch your speed and look farther ahead in traffic. you can always hop skid in case of emergency (which i can do now, i'm proud to say) or lock the rear wheel with your foot if your chain snaps. however, the chances of your chain snapping are close to Nil. if you know how to install and maintain a chain, the chances of it snapping are equal to those of your brake cable snapping.

the biggest obstacle toward riding brakeless, imho, is developing enough leg power to slow yourself and stop when going downhill.