I am a chemical engineer. I know this seems really good, especially in times of high fuel prices, but there is a catch. To split water into its components (oxygen and hydrogen) requires energy. You know this because we've all seen the school experiment where you hook up a battery to the electrodes in the test tube of water and the H2 and O2 are formed at the cathode and anode when the current is turned on.
So, here's the catch: When you run this type of device in your engine, you use some electrical energy to split the water, then you get the energy back in the combustion chamber when the hydrogen reacts with oxygen and creates the water again. There is no net gain.
But wait, it gets worse. The electrical power is not generated at 100% efficiency by the engine. There is some loss in the coils and from resistance in the wiring. And, the combustion process is not 100% efficient, either, not to mention the mechanical linkages to the generator, etc. So in the end you are loosing energy, not gaining in energy (mileage). In fact, it will cost you energy to run this type of device in your engine.
These types of devices are another variation on the old "perpetual motion machine" that were popular scams in the 19th century.