I tried it as a kid and it seemed to work for finding water mains, but it wasn't a controlled experiment by any means. It seems to me that wire rods will twitch in response to tiny movements by the person holding them, especially if you lean forward or backwards by a degree or two. A farmer I knew claimed to be able to use a pendulum and a map of her farm to find lost tools, and also told me that she had had problems with theft. Later a worker on her farm who had fallen out with her told me that he had tested her abilities by hiding a spade. She never found the spade. A few years ago, my neighbour attempted to use dowsing to find water, claimed to know where the water was and how far underground it was (3m). The location he chose was close to two existing wells, in a fairly lush wet area. With a group of friends he proceeded to dig by hand a completely dry hole to 3m and then declared that the water was actually 5m down. When he failed to find water at 5m he gave up. Incidentally one of the participants wrote on a blog that they had successfully found water. Eventually he hired a drilling company and sunk boreholes at two other locations. They had to go 40m down before they found water at one of the spots. He claims this location was found by dowsing, but it seems to be confirmation bias to me. The drilling company said they could always find water if they went deep enough.
