[teh_g]

I've got one from Bosch (a name brand), and use it to check for steel reinforcements in concrete and power wires in walls (and, rarely, studs). I can't double check the readings non destructively, so I evaluate the device according to whether in the end I hit something or not when drilling.

The readings are imprecise at best, with lots of false positives and possibly some false negatives (no way to know).

The tool is useful, if repeated readings are coupled with domain expertise, common sense and intuition.

If you ask me if the tool, by itself, works, I'll reply "maybe". All I know is that, aided by the tool, I haven't done any damage yet.

I don't think it's the tool's fault though. Rather, it's the combination of non-destructive testing and the limited feedback.