I tried it in my pick-up for the first time since i got it two years ago. When we flushed water with some detergent(?), there was indeed a lot of sooth that came out (not black smoke). I guess it'll be effective if you do it around once a year or more just to get rid of some of the loose deposits that have built up over time.

Of course, a properly running and maintained engine is still the most important factor as the others have mentioned.