The think the Prius uses breaking to charge the batteries also. I believe it works as a motor/generator by switching the field. While it has been stated you cannot get a NET GAIN in energy, this is different because instead of the energy produced and wasted in brakeing on a normal car (heat), switching the motor to a generator allows resistance caused by the generator to slow the vehicle, taking back some of that energy. Still a net loss, but less of a net loss. There are no extra parts other than electronics so i doubt weight is an issue.

Different than burning gas to load up the alternator to perform the electrolosis needed to split H2O. By the way, most hydrogen production is a result of reacting natural gas with high temp steam. Byproduct CO2 and CO. So those "no-emission" hydrogen cars get their fuel from another fuel we could use, while the production of that "no emission" fuel causes just as much greenhouse gas emissions (if not more when you add in the fact that something has to heat up all that steam, Bob and Hank drive their cars to the plant, energy put in for compression of the H2, sequestering of the CO2, etc.

Josh