I know what you're thinking. "There's no friggin' way this works."
Well, I have one. Not this particular brand, but a similar one. And it does work. They'll warn you that it may not work in houses with old wiring, but my house is from the 1800's, with wiring from the mid 20th century, and it works great. Just give it a try, this is a nifty little gadget.