I have been using STP fuel injector cleaner for a while in older diesel engine 4x4s usually between 10 years and 40 years old. My experience on these vehicles has been quite good overall,
The carbon content reduces, lesser smoke, marginally better pick but definitely better mileage. The older and less the maintained the vehicle, the better and more noticeable the results however you may need to repeat the treatment a few times depending on the condition of the injectors and fuel system.
If used on a new or relatively modern vehicle, 5-7 years the results are less noticeable.
I recently purchased STP fuel injector for a 38 year old petrol engine, lets see what the results yield.