They can do some pretty remarkable things, but results will vary from one to another and to the patients condition. I have used one at various times in my life.
I was talking today to a friend who has a problem with her neck. She went to a chiropractor who took xrays and said, "you will need a plan of treatment which will be 72 visits, and the price will be $xx. but if you wanna pay in advance we will do it for 1/2xx$. I told her to run, not walk to the nearest exit.
I have never heard of one of these guys being able to predict the length of treatment with such authority.

It is also amazing how many young children this guy treats. Don't ask me how I know. I just do. I can't reveal my source.