IMO if you are getting good fuel in your area, the additives will not help a great deal.
We do tend to have low cetane values in the U.S., but I have yet to see any measurable or quantifiable proof that these additives help me. I have yet to see any significant water in the fuel I have purchased (12,411.472 gallons over the trucks life) and my fuel economy stays relatively the same (with or without the additive). I see very little change in the presence or absence of smoke in the exhaust or carbon on the EGR valve with or without fuel additives .... assuming constant driving conditions and driving style. I have 194k miles on the truck. Turbo and injectors are original. I am fairly heavily tuned and have a tuned FICM.
I hand calculate my fuel economy on every tank, I chart it, I get the statistical average and standard deviation on the data. Driving style, towing or not, tire pressure, and wind speeds of 15 mph and over are far more important to the fuel economy then most anything else (except a dragging brake first identified by reduced fuel economy and then by IR temperature scan of the rotors).