David,
I am using the Ford Cetane Additive mostly as extra protection from the Ultra Low Sulphur Diesel. My dad has been using it in his Job 1 (built April 07) F-250 since the moment it left the dealership (literally). I usually use the recommended dosing suggestions when I fill the tank from 1/2 full to full. When the tank is below 1/2 full and depending on how low it actually is I may double the dosing just to be sure I don't dilute the Cetane booster mixture of the fuel that was still in the tank before the fillup. I've been using it for the last year at every fill up since I lost three injectors and an EGR valve, with no problems (knock on wood) since.
I'm pretty sure I gained roughly 1 MPG with the additive and the truck does feel a little smoother. Quieter engine, and more power, yeah maybe it's hard to tell.
My main reason for using it is because I have read numerous times that the cetane rating of diesel fuel straight from the station pump isn't really high enough for everyday diesels let alone high performance diesels. My understanding is that the ideal cetane rating for diesel fuel should be between 50 and 54 points. Most fuel station pumps that list the cetane rating have a rating 43 to 45. One piece of info I have yet to find is the number of "points" the Ford Cetane Booster Additive is supposed to add to the fuel's cetane rating. Another reason is that I figure a better performing fuel will have less particulate matter which in turn will prevent build up of soot in all the places that an EGR equipped diesel like my 6.0 Liter or your 6.4 Liter Powerstroke may have problems due to soot buildup and internal corrosion. Take this part with a grain of salt-I have been told by people at the local Ford dealership that International actually recommends an additive because they like to see the engines running with a cetane number between 50 and 54-but this certainly could be perceived as a sales pitch for a Ford labeled product.
Just my two cents with some extra theory and logic thrown in for good measure.