Short Answer: It's crap, as is any device that is claimed to boost fuel economy.
Long answer:
Even if it did work, which it doesn't, let's take a look at what they're claiming:
1. It's a capacitor. It stores electrical energy. This much is true, but the amount of storage available in a capacitor that small would be negligible. Your car battery stores 400Ah or more. The biggest 12V cap I can buy is about 0.1 Farads, which would run your car for a fractional of a second. It's a BIG cap, too - 5.25" tall, 2.25" diameter.
2. They say that if you install multiples the gains won't double. That's false. A capacitor put in parallel with an identical capacitor will, in fact, double the effective capacitance of each capacitor. If their overall claim is true (and again, it isn't) then in theory, you would be able to put in 100 of these and gain fuel economy rivaling a bicycle. The backpedalling on this point alone proves it's a scam - either you improve with more, or the effect is psychosomatic and they don't want you to realize you've flushing money down the toilet.
3. If it was capable of doing the things it claims to do (if it was made from a pure Unobtainium matrix) then it would be obscenely dangerous to provide to the general public. A charged capacitor capable of improving a car's efficiency would probably kill you if you touched it. You wouldn't do something as daft as touch both terminals on your car battery, would you?
If I were to invent something that actually could do this, I'd patent it then lease out the technology to every car manufacturer in the world. I wouldn't mess around selling them in lots online. Despite the tinfoil-hat claims, a working tech like this would be worth millions, if not billions, of dollars to the first company that could bring it to market in a vehicle. Everyone wants lower consumption, and if you can bring this kind of market pressure on your competitors, you would do very well.
