The Internet is full of discussions on these things. There is no scientific evidence that supports their usefulness, but lots of people saying they are snake oil.
I have a customer who lives in a very expensive gated community who has a 40 some foot sports fisherman boat. It's gorgeous if your into boats. He wanted to have a softener installed so that he wouldn't have hard water spots after he washed the salt off the boat. A neighbor of his talked him into one of those magnetic gadgets. He bought one for over $800.00 and brought it up for me to see. Even though it was an impressive looking gadget made of stainless and brass and had one heck of a magnetic pull, it didn't do one thing for the spots on his boat. I even called a friend who teaches physics at USF and had him explain why they don't work, but he had to see it to believe it.
Did I mention that I have a bridge for sale?