In reply to this and your pm so everyone can see it, I don't mean to brow beat you into submission. By all means make your own choices. I just believe this stuff is such a fraud.
There's two basic principles that this system, and there are many variations of it on the internet, are trying to beat. Conservation of energy and the first law of thermodynamics, quite well founded principles of Physics - both are about saying you can't get something for nothing (to keep it not too technical). In other words, for this 'system' you start off with water and electrolyse it using electricity from the alternator on the car, that takes energy (from the car's engine - by burning petrol), you then burn the gas, HHO, or Brown's gas as otherwise known in the car's engine (along with Petrol) and get some of that energy back in the engine - the process of electrolysis isn't 100% efficient, neither is the process of conversion of combustion to mechanical energy, so you're backing a loser from the outset, you can't even get back what you put in, some energy goes off in heat an fundamental ineffciencies of of an Internal Combustion Engine. That's the underlying principle, there's no mechanism for excess energy and therefore mpg improvement at the basic level, that really shoots it down for me without anymore argument. Tell me where the excess energy is coming from would be my challenge to these people.
However, just to indulge the argument that these people finesse it with, they talk about efficiency improvements in the combustion of Petrol by the addition of HHO (that sounds almost plausible as a mechanism until you look into it) and also go on about a catalytic effect of introducing HHO (they don't seem to understand what a catalyst is for a start - something that's consumed as part of the process is not a catalyst), I've even read some stupid 'theories' about cold (or cooled :shock

fusion and zero point energy to attempt to explain why this system miraculously works, but none of them do know why it works, none are scientists, none can prove it works under controlled conditions and all are either indulging in wishful thinking or a conscious fraud, it doesn't work. None of these slightly more sophisticated arguments stand up in any controlled test of this technology, there are some moderately controlled tests are out there by the American press and Automotive Societies for example.
They, in particular the American websites trying to sell and promote this stuff, have an awful behaviour of hitting every critical website and forum with anecdotes about how it works (fake ID's and multiple accounts and all that), and that is the basis of the scam. You get all your mates and so on to go on every website you can find and contradict anyone that says this doesn't work and make startling claims. If this was a genuine and workable scheme would it not be much easier to promote than they seem to struggle with? There's one particular promoter/inventor of this technolgy who despite all his high claims has always refused tests of his claims under controlled conditions.
And if you read more about the conspiraciy theories, inventors dying in suspicious circumstances and the technlogy being covered up or the patents being bought, I mean please, seriously, we can all buy this basically simple kit or designs for it and have a go. It's largely an internet myth - that's an opinion I grant you. Don't get me wrong, I'd love to think there's something in this that mainstream science has missed, I'm not that arrogant a scientist to believe that we know everything, I just don't see it in this particular 'technology' without evidence. Come back to my point, show me the evidence, not anecdotes from poeple who can't be trusted.
Equally, if you do fit it and can show genuine evidence of it working I'd be the first to be insterested, in fact I'll promise to promote it for or with you, but I think the likelihood of that is pretty slim.