I have seen these devices offered over the years, they come and they go away just as quickly. The UK with its prohibitive gas prices has seen its share of these devices, none of which seem to work. The tests that I have read indicate that there is indeed an improvement in mileage to the gallon after fitting one of these things, but that those figures are wholly attributable to the owner driving more "economically" to justify the expense of the device by helping it to work.
Reading the comments of his customers, they report in general a two or three MPG increase where they quote figures at all, and you can get that just by moderating your driving habits alone. Personally, I would stay away.