This is one of those topics -- along with most 'physical' things such as palmistry -- that I so wish someone with funding would finally do some science on. It could be worked out to actually measure these results for 'water witching' which has been done since the dawn of time and has an obvious physiological component in a way map dowsing does not.
I have used it to some effect in the past. When I was switching from well water to municipal water a fellow from the water department came around to do an initial assessment. Locates hadn't been done so he grabbed a couple of rods and dowsed where he thought the line would just to get an idea. He said something along the lines of "I hope my bosses don't see me doing this". Later I was digging to find the line that went inside my house and couldn't find it (I thought it would come straight out) so I grabbed a couple of rods and dowsed it and it showed it came out at an angle. I was dubious but when I dug there, that is exactly where it was. So it worked for me. Granted, it wasn't a large area, but it still found exactly where the line was.
It was one of many incidents that convinced me "There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.". When scientists, materialists or even wannabe scientists say things aren't possible without even trying them, I almost completely disregard them.