I read around and found that the theory is it "reduces surface tension of the fuel and vaporizes it better resulting in more complete fuel burn". The one truly good empirical test I found was a guy on a Saturn board who calculated his mileage exactly, knew what it should be, and maintained a constant conservative style of driving (some sort of fuel-efficiency nut who also bragged about getting 400,000 miles out of his rear brake pads...). He say absolutely no gain over a few thousand miles and a couple of months running various concentrations.

I didn't go much farther than that. In my opinion it's an old wive's tale and that any improvement is either in some sort of cleaning effect that could only really work once (and even this I doubt) or more likely a psychological effect, in that people are likely to drive more sensibly with the additive in the tank.