It makes sense that injecting hydrogen would improve fuel efficiency and clean up the emissions. Substituting an alternate fuel to replace some of the gas or diesel will increase the number of miles you can go on each gallon. However, producing the hydrogen on board via electrolysis requires electricity that must come from the alternator. That means a lot of extra load on the engine, which would be tough to make up from the hydrogen. The test results from the company don't give any details on the test procedures, so it's not clear if they were actually using power from the engine to drive the electrolysis unit. If they can actually produce hydrogen using less energy than the hydrogen produces, this could be a really benefit. Unfortunately, I'll have to see some real data before I put one of these on my car.
[Source: H To Go via GreenCarCongress]