This ones been doing the round since I was a kid and just past my test. Petrol was a mere £1.25 a GALLON
. all it is , is a magnet of some sort that allegedly changes the molecular structure of the cars fuel. PAH!! never in a million years. I remember Top Gear trying one in about 1990 if any thing it reduced the mpg due to the fitting cost (at the time). Don't waste yuor money. Keep the engine tuned and serviced regulary and try Redex evey now and then. 
:D
Oh and if it's that good wouldn't the massive petrol companies be all over it? Think about it.
(well, something similar anyways....)
)