UKPassats

Hi I was just wondering has anyone used one of these? I've put one on my car and I'm not sure if it's made any difference whatsoever, except for the fuel consumption, which has dropped Am I being unrealistic and expecting too much? I'm really not sure what to do because it seems like I've wasted my money. What sort of difference did it make to your cars? A massive one, or just a mild one? Hope someone can help, cheers.