Any thoughts on Mass Airflow meters?

I see that Performance Products has a 911 Mass Flow Sensor/software package with two graphs (horsepower and torque from 4000 to 6500 RPM) showing what appears to be fairly significant gains. In the print, they claim gains up to 19%. I've heard a lot of claims and am very skeptical, but am also interested in trying it ... if it works. Anyone have any experience with this or similar products?