Why Should I Use A Shock Collar?

I have heard a lot information about shock collars and how effective they can be when trying to curb a dog’s negative behavior. I have never actually used shock collars, but I have friends who use them on a regular basis. I would like to know more about these type of collars as a form of behavior modification for dogs. I wonder how many professional dog training schools employ these type of collars as a means of training their client’s dogs. I wonder how many people have found that these collars are safe and effective tools for behavior modification of dogs.