Best diesel treatment which most garages use is

I use 2 x shots of Millers Diesel Power Eco Max in every tankfull and I fill totally every week (500+ miles). I think I am getting slightly better MPG , no smoke and the engine is smoother but its one of those things - If I didn't use it would I have a problem and be worse off or is it truly doing good ? Personally for £12 a month and having had to renew an injector before I started using it and seeing the injectors condition I feel it's worth every penny