We've tried this product and it really does work - you can see the swarf on it if you have a look after an oil change. Certainly I've tried other similar products that don't work as well and cost more.
PS: You are supposed to buy a new one every oil change, but I just cut the filter open, clean it and pop it back into the new filter.
Below is the detail on the packaging:
Converts any spin-on oil filter into a super filter!
Removes 40 times more grit than a filter alone!
Helps prevent friction and premature engine wear
The most important improvement in engine preventative maintenance since the invention of the oil filter! Simply insert Magna-Guard™ into the oil filter. Magna-Guard, the most advanced ceramic magnet made, magnetizes the core of the filter to trap metal contaminates as small as 0.5 micron. Plus it ensures better oil flow and lubrication during the life of the filter. Virtually maintenance free and rated to withstand temperatures of 842° F. Insoluble in oil. SAE tested engine safe.
TOP ENGINE PROTECTION AND MORE
Increases fuel mileage up to 5%!
Doubles oil and filter service life!
Reduces engine wear 8 to 14 times!
Reduces tailpipe pollution by approximately 18%
Dramatically cuts overall maintenance/operating costs
Below is an excert from a review in Motor Trends magazine:
"Metal shavings can be the death of your engine. One way to avoid the trouble and expense of engine damage is the Magna-Guard, a magnetic metal pellet that slips into any spin-on oil filter. It sticks to the wall of the filter and traps metal particles as they travel through. Add a new one each time you change oil. We don't have the equipment to test Magna-Guard, but they'll provide SAE and independent tests that confirm it works, and does not inhibit oil flow."
And from Max Power magazine:
"Ever wonder what turns your oil from a golden amber brown to that icky black mess after a few thousand miles? A lot of things, but one of them is metal particles. Even under the best driving conditions and the most meticulous maintenance, your engine will shed a little metal. Over time, these particles can work their way back into the engine and cause wear on a number of key components such as valve guides, piston rings, and connecting rod bearings. An economical way to extend tile life of your engine is with the Magna-Guard filter insert. Just drop it into your filter when you change your oil, and replace it at each oil change."