My conclusion (based on lots of miles, and lots of different additives tested) is that if you are looking for increased mileage / fuel savings to justify the expense - save your money.
However, the cost of fuel injector replacement in most modern diesel engines is high. It seems plausible to me that a fuel treatment regimine can extend the life of fuel injectors, and in the case of certain types of injector failures, even your piston crowns. Have most definately seen additives WORK to free injectors that were sticking and decreasing power and mileage. free up, clean up exhaust, and return mileage to normal.
Most over-the-road drivers swear by the stuff. Howes, Lucas, FPPF, Diesel Kleen...who knows...makes you feel good when you add it and go down the road, that I know because I have burned all of them at one time or another.
Never found one that measurably increased mileage. Never found one that wasn't expensive. Never found one that grew new hair on the top of my head.
Personally, I like Howes. Can economize when rolling on lots of miles buying a big jug, its formulated to be added in greater quantities / gallon of #2 than some other products, and to me that equals more likely to get correct and consistant mix when treating ~34 gallons of fuel at a time. Howes looks, smells, and feels like something that should burn in a diesel. See lots of it going into the big rigs. Seems to keep my Duramax injectors happy - but again, who knows.