Okay i'm at home wondering about this, i found an articale in wikipedia:
There might be merit to HHO, but not as the way they advertise... 50% is probably based on higher concentrations of Hydrogen, than what HHO generators can produce. But that small about might still give an improvement.
This part got my attention:
Automotive fuel enhancement systems work by injecting a hydrogen rich mixture, or pure hydrogen into the intake manifold of an internal combustion engine combined with air/fuel ratio, and timing modification. A small amount of hydrogen added to the intake air-fuel charge permits the engine to operate with leaner air-to-fuel mixture than otherwise possible. As the air/fuel mix approaches 30:1 the temperature of combustion substantially decreases effectively mitigating NOx production.
Under idle conditions power is only required for extraneous components other than the drive train, therefore fuel consumption can be minimized. A 50% reduction in gasoline consumption, at idle, was reported by numerically analyzing "the effect of hydrogen enriched gasoline on the performance, emissions and fuel consumption of a small spark-ignition engine".
Under most loads near stoichiometric air/fuel mixtures are still required for normal acceleration, although under idle conditions, reduced loads, and moderate acceleration Hydrogen "addition" in combination with lean burn engine conditions "can guarantee a regular running" of the engine "with many advantages in terms of emissions levels and fuel consumption".
Increases "in engine efficiency are more dominant than the energy loss incurred in generating hydrogen". This is specifically with regard to use of a hydrogen reformer. Overall computational analysis "has marked the possibility of operating with high air overabundance (lean or ultra-lean mixtures) without a" substantial "performance decrease, but with great advantages on pollution emissions and fuel consumption".
Overall comparing "the properties of hydrogen and gasoline, it is possible to underline the possibilities, for hydrogen fueled engines, of operating with very lean (or ultra-lean) mixtures, obtaining interesting fuel economy and emissions reductions". The "concept of hydrogen enriched gasoline, as a fuel for internal combustion engines, has a greater interest than pure hydrogen powered engines because it involves fewer modifications to the engines and their fueling systems".
 Engine Control
ECU or carburetor modifications are required to establish lean or ultra lean burn engine conditions; where the hydrogen permits leaner conditions than possible with solely hydrocarbon fuel.