I used to ride as a messenger in Vancouver. Singlespeed (but with a freewheel) rocked!
Nice strong wheel, cheap, never miss a shift, always in the right gear, light. I could replace my whole drivetrain for under a $100.
Just make sure to spring for a dcent quality BMX freewheel. The cheap ones self destruct, and will throw you to the ground, unfortunate if you happen to be sprinting a cross a busy intersection...