It seems to be pretty good at reducing smoke, but I can't point to any measured MPG gains. Certainly NOT worth the $400+ it costs.
Live and learn.
That said, I get 16 city, 20hwy all the time with big injectors and a 4x4, so maybe it helped some. I installed more than one thing at once, so I can't point to any specific before/after for the Fitch.
I'd highly recommend it on GAS engines-- but on a diesel, I don't think it's worthwhile. (I have one on a gasser and it made a very noticeable improvement in mileage, emissions, and power)