Didnt do much of anything for me, but didnt hurt either. From what I've heard its useless in ethanolated gasoline. Although I havent even gone through a full tank for what thats worth.
My guess is that itd be more useful in a dirty engine, or if something is making you run rich.
Supposedly it works by breaking the surface tension of the gasoline...in other words, the molecules stick together less, and they can be more completely combusted. Which I dont doubt considering how polar acetone is. But ethanol is polar too, and that doesnt exactly make gas mileage any better. I suppose that has more to do with the fact that ethanol isnt as energetic as most gasoline hydrocarbons in the first place.
My guess is that itll help most people out by cleaning their engines out. If youre using ethanolated gasoline, its almost likely a waste for sure.
On non-ethanolated, it could make more sense. If theres nothing else already in there polar enough (is there? not sure) to split everything up, I can see it helping. And Im not even positive that a fully maintained and efficient engine isnt burning pretty much everything up unless theres already a problem with it to begin with.