Unfortunately, the water molecule is very stable and doesn't like to be broken apart without considerable input of energy, much more than can be found in a regular automotive engine. It takes much more than just coming into contact with a platinum catalyst. On the other hand, the individual hydrogen and oxygen molecules like to combine back together into water, sometimes rather explosively. And since a regular gasoline engine isn't designed for hydrogen injection, it would probably have bad consequences on your engine. While you need to make your own choice about this product, my take is that it won't live up to it's claim and may well end up costing you a lot of money in engine repairs. (By the way, HHO is just a fancy way of spelling out water (the chemical formula is H2O.))
Having said this, this really doesn't much sound like a work-related question. More like a general interest, should I spend $50 on this? Eng-Tips is really designed for Engineering Professionals who are asking work related questions.