Have to cut filter open to find what you have collected - very messy.
It will only collect ferrous material. It will show when the tappets,
cams (cause = top end spray bars coked up) or crank thrust faces are
wearing. It won't hold bits of alloy piston, thrust bearing faces or
fatigued big end bearing overlay. EVERYTHING you find has already gone
though the oil pump so that's trash.

No even good in principal. What do you expect it to hold on to? Anything
large and iron will be stopped by the filter element. The filter is after
the pick-up screen in the oil sump, the oil pump, and the pressure release
valve. The oil pump pick-up screen should stop the vast majority of any
large iron parts that might heve broken off of the crank, rods, or valve
gear. The filter element should handle the rest. If you want to add a
magnet, add it in the sump. A magnetic drain plug is much more useful than a
magnet added to the filter. A magent in the sump can hold onto any chunks
and keep them from bouncing back up in to the moving parts.

Ed

Better to get a magnetic drain plug...  Wipe it clean when you remove it.
It will catch only ferrous metals though...  If you want some real
protection, get a dual filter adder that not only gives you two filters
(parallel, not in series) but also an extra quart of oil.