So this place is a bit of a misnomer. There is a research library here if you are interested in actual research (of course, call ahead to make sure they have what you are particularly interested in, just like any research library). This is also where McKinley is buried (as is Ida and their daughters) in a huge, very Victorian, mausoleum.
But a museum on McKinley? Let's say that's a stretch. There IS a gallery devoted to McKinley, but that's it. This is a kids' museum, complete with dinosaurs and live animals. There is nothing wrong with it, but be warned! We actually took my three year old to this last month to give you an idea. I was very surprised when I first went a few years ago (I was also not personally impressed by the gallery, but I probably know too much).
If you want a good museum on McKinley, either go to the Saxon House, which is devoted to First Ladies and Ida McKinley, or go to Niles, Ohio, and tour the Birthplace Museum. I have not actually been the birthplace museum (on my list) even though it is less than an hour away because it has terrible hours so call ahead if you want to go.