[10ren]

If it stored energy when braking, it would be more efficient.

But on my bike, I only really brake in rare situations, for safety reasons (cars); unfortunately, friction automatically supplies adequate braking for me.

I also choose paths that minimize braking. I guess it's possible that that might change if I had a bike that efficiently stored/applied energy, so that it was as if the braking hadn't occurred. But I don't think so; for one thing, it would be no where near 100% efficient.

[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.