The simple physics of this idea make it impossible to achieve any positive results in my opinion.
Basically energy is a constant, so the energy contained in the produced HHO needs to come from somewhere, in this case the electricity passed through the system. The energy for the electrodes then needs to come from somewhere, in this case from the alternator of the car. The energy to make the alternator work needs o come from somewhere, in this case the engine turning the appariate belt. The energy for the engine to turn needs to come from somewhere, in this case the fuel that is fed into the engine.
So assuming you achieve the impossible and get a 100% efficient energy transfer at every stage of the process you will only get as much put back into the system as you get out of it. Unfortunately you will never achieve 100% efficiency as you will loose energy from heat and noise in all stages of the process and so it will reduce your fuel efficiency.
Personally I get the impression that this is a large case of snake oil, backup up by fake rumours concerning the invention of the system in the first place. The fact that most kits available commercially come with baking soda makes it even more suspicious as this will produce CO2 and not HHO (or more technically HHO2) which will kill your engine efficiency as you will be replacing the air flow into the system with a gas which inhibits ignition.
If you want i can give you the complex chemical explanation of how splitting water from 2xH2O into 4xH and 1xO2 will never produce as much energy from the H as you put into the reaction but it does get rather boring unless you are a chemistry buff (trust me, I have just had to sit through it form my other half

)
'99 3.2V6 Auto Limited.
Some day it will be how I want it to be...