Has anybody ever used one? How did it work for you?

I have replaced 3 shafts on my 25 merc and the one I am running now is bent as well.
I have gone through more props than I care to count, the stumps at Reelfoot have taken there toll on my motor and I am ready to try a prop guard of some kind.
I like the looks of the Mac's River Runner the best.
I am just wondering if they work and how much it will slow my boat down??