I've used one of these types of units on two different trucks. They didn't work on either one. We put on a gauge that measures mpg on both of the trucks, and the fuel savings weren't there.
What generally happens is someone puts one of these devices on, then alters their driving habits. They don't drive as fast, or they don't take off as quickly, etc. That gives the fuel savings. I can get 10% better gas mileage than my wife does in the same car. Has nothing to do with fuel, just the driver and techniques.
My truck is getting about 20% better gas mileage now than it did when new. In that time, they added 10% ethanol to the blend, which should yield lower mileage. I did change some driving habits in that time.