re: Stud Finders ?

Has anyone ever tried one of the sensors you see commercial for on YouTube where it connects to your phone to show you whats behind the wall (pipes, studs, wires and even a rat)?  I think it is called a Walabot.  It is supposed to be able to see 4 inches deep behind sheet rock.  I think it only works with Android phones.  

They look pretty impressive in the ads but I am skeptical that they work as well as they show.