I belive this thing is in the market for a long time, and I have one of this, it doesn't seem like it is helping my nsx.
However, according to my brother, it helps a little bit (about couple mile more per gallon) on his Ford, but it only helps when he drives on the long trip on freeway without stop. He kept track of the gas mileage when he traveled back and forth from San Francisco to UC Davis. And he is a mechanical engineer now. So I believe him.