I used to crewchief a pro stock motorcycle. It did not use an oil filter but had a strainer on the oil return line. I epoxied rare earth magnets to the outside of the strainer. There would be some magnetic debris caught by the magnets occasionally. I checked for debris after each race weekend, probably 15-18 passes down a quarter mile track, when I changed the oil. So yes they do work. I have a oil filter cutter to remove the outside can so you can see what is trapped in the filter media on an engine with a common oil filter. If you can stop metal pieces from circulating, that is a good thing.