These things have been debated for years. There might be a reason they aren't used by an automotive manufacturing world desperate to come up with ways to improve fuel economy.
1. Electricity isn't free: no energy comes from nothing. One of these units eats 900 watts. A 900 watt drain on your electrical system has to get power from somewhere... like your engine.
2. If you go to electricsupercharger.com you will see one of the oldest and best established of these kinds of units. The owner claims almost the same power draw and only 1 psi of boost and if you've ever seen one of these things run, you know that the 5psi claimed on EBay from the same wattage output is pure nonsense.
BTW for a 5.7 or 6.1 motor the electricsupercharger guys say you need two of them inline. 1800 watts on the motor. No problemo.