At first thoughts I thought they were a con.. BUT (and this is with outside viewpoint cos i dont have one).
BMW Mag tested one and said it worked, as have a few other big brand mags. People say they are noticing better MPG. In a car without an on board computer I can see where the "I want it to get better MPG" factor comes in, but in a new Clio with a fuel computer the figure is right in front of you, if youre doing 10MPG better, itll show it on the computer.
I might give it a try at only £70. If I dont like it they always give money back!