My main purpose is to use it as a fuel stabilizer, since I live close to work, my car only gets used on the weekends. I do notice that during cold engine operations, or on colder days the car produces less white smoke. If indeed the engine burns cleaner during cold operation when incomplete combustion produces acids then I should have less engine wear due to corrosion. Other than that, I do notice a mileage increase, on a 220mi drive non-stop, I now save a 1/8 of a tank of fuel, so the car either runs more efficiently or the combustion chamber is much cleaner. As for the smoothness factor, it is hard to say whether it is there, my main purpose for my situation is fuel stabilization.
Another observation in 2-stroke operations, cold start up produces less smoke, so this product must be helping oil separate from gas before full engine warn up to lubricate instead of being burned up. 2-stroke is primarily fuel stabilization, I do have smoother starts and cleaner plugs on my 2-strokes as well.