These kill me, it dawned on me what is the most troubling thing about these. If it provides a benefit, than it must be generating a thrust.
Since it is in the airstream, no matter how aerodynamic it is, it will be an aerodynamic drag. So not only will the resultant thrust created by this thing have to overcome the drag it creates, it will have to generate additional thrust in order to provide a benefit to the car upon which it's mounted.
Does that make sense? It would need to generate a substantial amount of thrust, and I don't believe it possibly could generate any thrust.
This is the aerodynamic equivalent of an HHO generator.
Website claims are 100-200% fuel mileage improvements.