Sorry, but since this is my thread, I'll chime in...
There has been absolutely zero evidence that on-board hydrogen production systems work; the energy used to produce the hydrogen is less than what the hydrogen is capable of producing (hydrogen actually has a relatively low capability of energy production). In other words if it were to work, you'd have built a machine that produces matter; impossible.
All evidence points to the mileage gains from a key part of these "kits"; the part which modifies the signal from the O2 sensors. It forces your engine into a lean condition, there by using less fuel. At the same time, causing excess heat, wear and the possibility of detonation.
Think about this for a minute; the contention that having an HHO system installed would push excess oxygen into the exhaust system, there by requiring the need to trim out your O2 sensors is absolutely ridiculous. Water is made up of two molecules of hydrogen, one molecule of oxygen. If you split the two, you'll either make a gas containing two parts hydrogen and one part oxygen... or a gas containing one part hydrogen and one part hydrogen-monoxide (HO). Either way, you're producing a combination which provides two molecules of catalyst for every molecule of oxidizer; to create the reaction that is flame, you generally require much more oxidizer (READ, oxygen) than you do the catalyst.
Most people know that IC engines biggest challenge is getting enough oxygen; not getting enough fuel. So there for, the idea that "excess" oxygen can be found in the exhaust... because it isn't "needed" is totally ludicrous (also, this would mean the engine is running lean). This argument, and the fix for it would be to add more fuel so the oxygen is burned and the lean condition is fixed. If you're adding more fuel to prevent a lean-out condition (which would be required if you're pushing large amounts of oxygen out the exhaust)... you surely are not making your engine more efficient. You would be creating more power, but that's about it.
Think about it like this... NO2... which everyone loves... uses one molecule of nitrogen (an inert gas) and two molecules of oxygen... if you can run NO2 in your engine and not have "excess" oxygen when tuned correctly... how in the world do you explain pumping one less molecule of oxygen but two additional molecules of hydrogen (a catalyst... NOT inert) in place of a molecule of inert gas and suddenly now you have "excess" oxygen?
The answer to all of this is... HHO is a huge load of BS; The O2 sensors are NOT seeing excess oxygen and being adjusted to counter that... rather, they are being adjusted to see a fuel rich condition (which does not really exist), there by cutting back the fuel curves and resulting in a fuel savings. Plain and simple, on board hydrogen production systems DO NOT work; the amount of hydrogen they produce is minuscule and doesn't effect anything.
And lastly... If these systems worked, wouldn't someone reputable be producing them, with purpose built, professionally made parts instead of mason jars, tin cans and compressed air filters normally seen in a shop's air system? Lets not even mention how sales would increase exponentially for a manufacture who's truck was capable of 40mpg from the factory.
If it sounds too good to be true; it is.