Do you know how the device works?
Most people don't know that like milk, gasoline spoils with time. This is why it is recommended to drain the gas from a car that has been sitting for two years rather than try to run it. What happens is that the hydrocarbon chains break down into smaller and larger chains, making for less than optimal combustion. The catalyst uses a platinum alloy to reconstruct the fuel by breaking down the long chains and building the short ones back up.
Ever wonder why people complain about "bad gas" when they fill up at some little po-dunk gas station? It's because the gas in their underground tanks has been sitting for too long.
As someone who actually goes out and purchases a lot of these controversial devices and does testing, I've discovered some that work and some that don't. The Fitch system works. The Tornado and hydrogen peroxide injection system don't.
Note: These claims are only valid for Marks. I can't stipulate regarding other vehicles, though my testing of the Fitch in our H2 also showed positive results. (For those interested, mileage values were taken before and after installation...the computer being reset prior to data collection on both vehicles.)