Besides guns, parts, and associated accouterments, they have a HUGE selection of books for the BP enthusiast ranging from gun building to historic novels and everything in between. The whole upper floor of the main building is full of books of all sorts. I spend about as much time pursuing the books as I do gun parts and such anymore. They have a huge collection of original firearms, mostly Ohio made percussion guns with several types of flint military rifles and muskets. Really nice powder horn, flask, and powder can displays too. They even have a shooting range out back if you want to burn some powder.
I get up there about twice a year and always plan for about 4 hours of gawking. If you are anywhere close, it is well worth your time to stop by. As jaeren said, really nice people.