This unit was actually of proper design compared to the other brands. Inside the solid steel case is not only a simple air passage, but a mechanical 1-way valve can be found. It allows a preset amount of air to flow in during induction, but here's the good part, the valve shuts off any back flow during off throttle conditions thus allowing proper metering of air.
The result? I'm now doing almost 400KM on the same amount of fuel and power is slightly better due to leaner burn. I'd venture to say my install worked well since it was used as a means of correction (correcting my "run rich" condition) but I'd dare not vouch improvements in power or fuel savings on healthy engines. Be warned :P
I'd recommend this but its' too expensive (from my point of view - mine was free) to justify the install, good as it may be (for me).