Injector cleaner works on Jaguars, no reason it would not on Mercedes. I suspect unless you have a problem with dirt in your fuel, there would probably be little benefit before about 10-12 years of age on the engine. How often depends on how often you have an injector hang up--no placebo effect here, you can hear and feel the difference when the car is at idle.

I use STP injector cleaner on the jag, plus Valvoline or Techron fuel system cleaner 4 times a year or so. Just the fuel system cleaner twice a year on the Cadillac. And Rotella DFA seems to do the job on the Benz, although I have tried STP diesel fuel injector cleaner there too, but have observed no difference. Sometimes I add it if I haven't had the chance to put the Rotella in for a while since the Rotella is supposed to be added to an empty tank, which means I have to have the bottle with me when I fuel up. The STP products get added to full tanks per the instructions, so are a little more convenient.