Those do work, but not great and they tend to have alot of problems. I bought one a few months ago and had to take it apart once already to readjust the internal sensitivity. Maybe ok as a back up but I would highly recomend the Garrett Pro Pointer. Yes it is over a hundred bucks but worth it.
I bought 3 of the Harbor Freight Pinpointers. Had so many frustrating problems, I bought a Garrett Pro-pointer and am very happy with the pro-pointer. I use my Harbor Freight pinpointers for what they were intended for -- finding nails in boards, before I send them through the planer..
i have one and use it hard, i've also done a few things to mine to make it last(changed out the power button with a more sealed one) for under 20 bucks ya can't go wrong. i also replaced that loud speaker with a vibrating motor, now i don't get as many strange looks at the park