Those things are sometimes useful for pulling out simple dents. They cannot pull out dents that have folded or crushed the metal though. And I am sure you are aware that if the paint is scratched, it will still be scratched after you have pulled the dent.
The types of dent that can be pulled with one of those plungers are usually found in the middle of doors, where there are no folds or lines in the bodywork of the car. You can usually push those type of dents out from inside the door without too much trouble.
All in all those plungers really do not do much to save you any money.