In short, they simply transfer power to the wheels with traction, when your car breaks loose. That is what is supposed to happen anyway. With a regular diff, you get your power going to the wheels without traction, therefore losing more and more of it. It's definately a noticeable change after you switch, but its a matter of preference - some like it, most dont...
Personally I think its a nice way to throw away money - but hey, if you've got enough to do that, why the hell not give it a shot?