I installed one awhile back, and I do believe they work. Last time I changed my oil, I found small metalic slivers on the magnet. Glad they are no longer in my engine. I even installed one on my wife's Volvo. Cheap $5.00 insurance.
I use them and the results are great. Everytime I've changed the oil, the amount of metal gets smaller so it is slowly cleaning it out. I also use the magnetic disc attached just behind the drain plug; I remove it before draining each time to let the trapped particles flow out. Whoever thought these up is a genius.