About 7 years ago, shortly after I moved here (WV) from NYS, I had to relocate an electric panel that was inside the collapsed barn. I knew there was a water line nearby as well, and figured I had to be prepared to turn off the mains, dig all around the box to find the power and replace whatever we damaged in the process. No one knew anything about the location as the lines had both been in place since about 1976 and everyone who had any part in laying them was long dead.
My neighbor pulled some coathangers out of his truck, cut and bent them into a good shape and dowsed the location for the lines, but couldn't tell how far down or whether both lines were there. He immediately called his cousin who, in his words, was "good at witchin' burred war and lines"
Paul arrived in about 10 minutes. He was retired from the phone company and we had a nice chat. He pulled out a similar pair of bent coathangers only his had wooden handles. He quickly located both the water line and the electric line, in the same places my neighbor dowsed. He told me that the water was down at 3.5' and the electric he felt was above it by a foot. They shared the trench in some places, then split off. The water line would not be disturbed by our work if he was correct. He marked the ground with some paint. He explained that dowsing was easy and let me try with his wires. I also had them cross in the appropriate locations despite the fact that I was sure it was nonsense.
We dug, and both lines were exactly where Paul said they would be. Whenever I have to locate some of the many thousands of feet of buried water lines here, I can dowse them and I have found lines in unexpected places many times. This is pretty convincing. Often the lines are not in the location you'd expect from common sense, so it's not as if the dowsing confirms an existing logical guess.
When trenching for our outdoor wood boiler's buried lines, I called the power company to locate the buried wire for my service entrance. I had dowsed it, but came up with a really improbable route. The power company guy came and located the line in exactly that location, and when we carefully dug to get under it, it was right there.
So yes, it definitely works.