It's mostly placebo effect but...
It depends greatly on what particular kind you have in mind. There are different types of cleaners, boosters, antigels and so on.
(Octane boosters just could not work because you'll have to put 5-10 liters of the stuff to the 50 liter fuel tank to get boost of 3-5 octane points)
I've used Redex fuel system cleaner for petrol engines (small white bottle with long neck) several times. I think that there was some improvement in engine performance (even idling and better work of cold engine). Also there was some improvement in fuel consumption.
But it's all for petrol engine. Have no info on Redex for diesel.