All it is; is a capacitor. They have these things all over the web called "Voltage Stabilizers". It's been on the "ricer" market for years. This one is just one of the more convenient designed ones you can plug in.
You can make one for a few dollars, and see if it works. In theory if you have a really crappy electrical system it can clean it up. It'll work the same way an audio cap does and clean up the radio but getting more MPG that's still to be proven. Most of it is placebo effect.
Good free way to better MPGs is a lighter foot. If you want you can add more PSI to your tires to reduce rolling resistance.