Not a lot of point to them in these engines I reckon. The casings and pistons are all alloy so after run in the only worn parts it'll catch are bearings, gears etc - and if THEY'RE wearing you've got bigger problems than a few bits of metal floating about.
Still there's no harm in it and it's cheap enough so knock yourself out.
FYI you can achieve the same result for free if you have a dead hard drive - pull it apart, get the rare earth magnets out, and stick them to the bottom of your oil filter with some decent glue.
