Less like your typical museum and more like a really really rich man's home (which it was) filled with great paintings, sculpture, and furniture. If you have been to Boston's Isabella Stewart Gardner Museum then you have an idea what this is like, except this house was actually used for living.
Unlike the larger museums, this is one site where you feel like you haven't missed anything.
Get the acoustic guide (free with entry) and listen to the experts describe the rooms and their contents.
After you are done, cross the street and relax in Central Park.