I say intermittent, because I only used it during times when the price of gasoline exceeded $3/gal. And during those times, I did notice significantly improved mileage
I cycled three tanks without to start, then three tanks with, followed by three tanks without, all summer long and into the fall, in 2008. Same commute, same car, took pains not to feather the accelerator or otherwise change my driving behavior.
It did improve gas mileage. The effect, whatever it is, persisted and slowly went away over the course of two tanks without. Third tank was back down to “normal” mileage for the commute, which was 34 mpg. I got it up to 38 with acetone, 3 oz. per ten gallons, always at fill up.
I still have the car, by the way, and it’s running fine, no issues whatsoever. It has about 40k miles on it now, synthetic oil, K&N drop in filter.
Not a whole lot during the period. All city driving on much the same routes. The first note was that frequency of fill ups was a lot less often. That much was obvious right from the start. All told, I figure mileage improved by about 1/3rd. That impressed me.