I have a few for Nevada. They have helped some good, some bad for me. Good things are they have sites listed that may or not be listed on newer maps. Bad is the older maps are usually not very accurate, and when you do the over lay, things can be way off. But it could just be for Nevada, as finding accurate pre 1900 maps can be iffy. Usually the Northern half is pretty good, but the Southern half can be a maze. But just having the names of a place and about where it should be has helped me find them on a state Atlas or similar map.
The one I have for gemstones has been interesting though. It's given me more hints than some of the rock hounding/gemstones books I have.