My dad uses one on longer trips, he claims about a 1 mpg improvement. He's a fairly detailed engineer and I believe him. He's been able to compare it on many long trips. His is bigger and is for his 5th wheel, it is a pretty big curved aluminum model. He was told to adjust it to keep about 1 foot of bugs on the upper part of the trailer and that is what he does. I think most of his testing was with a 6.0L Silverado and a fairly tall and vertical Cardinal 5th wheel. He now has a Duramax and a more aerodynamic Cougar 5th wheel, he still uses it on longer trips but I don't recall how much testing he did with it on and off.
