I can see where, properly placed, they could make a MINIMAL (less than 1% at a guess) difference in fuel economy by disrupting the airflow over the car.
That said proper placement will be difficult without your own wind tunnel.
I'm on it because of who I worked for in the past.
Huge pain in the butt. I can drive to Dallas from Wichita quicker than going through all the crap at the airport.