Seeing a famous painting in real life is like seeing it for the first time. You can see it exactly as intended, look closely at the brush strokes, and see the artist's mark on the work. Even the most faithful high quality digital capture still contains the opinion of whoever balanced the colour and contrast.
I saw Nighthawks today and it's quite unlike any image of it I have ever seen. Even the postcards and prints they sell at the art institute of Chicago are a little too vibrant and saturated, with the yellow of the cafe interior overpowering the cool exterior.