I'd expect the magnetism would throw any compass off..even in the outdoors. I'm not sure why you'd want to pay $25 or $30 for a biner to drag through a cave (or for any use, for that matter!). I think the main focus of the Magnetron is for guides to use it with clients who might forget or mishandle screw-gate or similar lockers (which cost half as much each). If your question is reliability, however, Black Diamond is a primo brand and, having used their gear since it was Patagonia and before, I'd give five stars to their product quality. A posting I saw a while ago on Midwest Mountain Guides gave it a thumbs up...although they note they did not use it in snow or wet conditions (and wet is exactly what we'd see in caves).