I don't buy it. (Black) Coffee is good for you, but I feel very dehydrated if I don't switch back to water after a few cups.
The simple truth is that, unlike things such as grain alcohol, when you drink coffee it puts more water into your body than it takes out. You might feel a bit dehydrated compared to drinking just water, but that just reflects how water logged you are used to feeling. Excess salt, lack of exercise, and other things can cause the opposite problem and make you retain too much water and then feel dehydrated when you drink a cup of coffee.
