I've been using one for a number of years and find a small improvement in mileage. It's worth the cost I paid, less than $300. My costs have been recovered long ago, and the rest of gravy.
When the bug line rises on the trailer, it means the bugs are following the wind carrying them. That means the trailer is in the slipstream to some degree, which improves fuel mileage. Sure it would be better if it could be closer to the trailer, but life is full of compromises so we do the best we can. Mine is on the back of my campertop as close as I can get it to the trailer.
