I use to have a Toyota Tacoma truck that would idle real rough. Every two or three months I would add some fuel injector cleaner to a tank of gas. The engine would smooth out. It made a believer out of me.
Now I add some to my motorcycles/scooters along with fuel stabilizer and a little octane booster. All run good.
Does not seem to make much difference in performance but, engines run nice and smooth. I assume it removes carbon from the spark plug and valves.