I recently reconsidered my regenerative braking for bike project - you can now buy 600F 2.7V caps for about $15 now.  that's 2.2kJ/100g.  For comparison, a bike travelling at 20mph (in the middle of the road) has 4kJ of kinetic energy.  So we should be able to store and recover a stop and start in say 250g (half a pound).  This would make the US back streets, with their endless four-way stops much more practical for bikes.  For the motor/generator we are not looking for continuous power, instead it needs to be able to cool off between each stop.  So we might be able to use a 4 times smaller motor (assuming 3 times derating and pulse operation).

The user interface would be quite simple - when applying the brakes the generator would have priority, when the user applies pressure to the pedals the motor would kick in.