Using the Tornado feul saver on a Deville

Has anyone here ever put one of those Tornado feul savers on their Cadillac Deville? I was thinking about buying one to improve my gas milage but I want to know if it will make a difference first. It cost's $70, that's worth it in the long run if it actually does improve gas milage but that's too much money for me to throw away if it doesn't do any thing.
 
So has anyone here ever used the Tornado feul saver? How did it work for you? How much did it improve your MPG?
 
 

I have not used it myself, but have been told that it is a gimmick.  I wouldn't spend my money on this until you get some input from here.