[ZeroGravitas]

It's only more efficient if it regenerates enough energy from braking to compensate for the extra weight you're carrying which is highly unlikely, particularly extra weight on a wheel since that requires 1.5-2 times the energy to shift compared with the same mass added to the frame (http://en.wikipedia.org/wiki/Bicycle_performance#Kinetic_ene...).

You might still benefit if you need an electrical boost provided by energy from the grid but normal bikes hit a pretty sweet spot, so I'm thinking this is mostly gimmick.

[10ren]

Oh gimmick it is. It's just that the spring-sprung-brake concept has fascinated me since I was 5 yo. It would be cool to make this work; if sufficiently light, it would be a net win.