Yes, they are great!
Gas tank additives are wonderful for STP, Gunk, and all the other fine manufacturers of these beloved little better-performance-out-of-a-bottle products. The stockholders of these companies surely love the high profit margins.
As far as our cars, well, IMHO, no. Gasoline today is blended with a number of solvents and other chemicalia to get rid of junk and buildup on injectors. So additives from the Pep Boys shelf won't hurt your car... but probably won't really make much of a difference either... :shock:
Over the years, every once in a while I see an article by one of those consumer-advocate and/or automotive publications, doing a test on various brands of gas additives. Invariably, no difference is noted between the "before" and "after" controls. So, cheepskate that I am, I avoid them, and my cars run just fine

...