'when used correctly'
The majority of people DO NOT KNOW how to use these collars correctly.
The following article is useful to read (but you must read it all the way through). This trainer says at the end that the prong collar is not her first choice of collar and that she rarely uses one (when she does it is often fastened in a way that it cannot tighten) and she NEVER puts it high up under the ears of a dog.
http://www.flyingdogpress.com/prong.html
I would not advocate the collars use, as with the shock collar, I feel that only a very small minority of people could use them in a correct way to be effective.
I feel that more harm than good could be caused by using them in the wrong way on the wrong dog.
As the article says, you should not have to use them for any length of time with a dog.
Training, building a bond with and understanding your dog is a far better option.