This has been discussed at some length in my local Subaru forum. There were a couple of other links posted as well. You have to be pretty exact about how much you add, but you'll see nowhere near a 35% increase in fuel economy.
I've been using it for about 5 tanks of gas now. The first three, the gains were zero to minimal. The 4th tank, I got some HORRID gas mileage. I don't know what caused it - bad gas, too much acetone, etc. So I told myself, I'll try it for two more tanks, and if I don't see an improvement, I'll just quit it.
I WILL say, however - I feel very strongly that adding acetone helped me pass emissions. I'm running a custom tune on my WRX making a decent amount of power, with only one high-flow cat (WRX's have three from the factory). I expected my emissions readings to be close to the limit....nope! I passed with flying colors. Acetone supposedly helps the fuel burn cleaner and more efficiently, and THAT part I believe.