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Chain drive can be up to 98.6% efficient, you'd have a hard time beating that.

http://pages.jh.edu/~news_info/news/home99/aug99/bike.html

I'd think that a pneumatic bottle would have to be very large, and/or very heavy to store enough energy for a useful return, the heat lost from the compressed air would lower efficiency, and an air turbine that can provide usable power over a wide range of pressures from the bottle would add additional weight and contribute to inefficiency.

The fact that there are few compressed air hybrids is probably a good sign that it's not significantly better than batteries.

Few electric bikes even use regenerative braking, it's generally not worth the weight and complexity.

Its much more efficient to store the energy as electricity. Compressing Air, and using a pneumatic motor has about 5-7% total efficiency.