I wouldn't do it with a mystery show where the surprises would be spoiled, but some shows I can happily watch over and over. I have seen Black Books several times and it still cracks me up. My husband and I have rewatched Firefly and Farscape after a bit of time had passed and we'd forgotten some of it. We also recently rewatched the first season of the X Files and it was interesting to see how well it had aged after nearly twenty years. I also sometimes use shows I have already watched to practice my German (switching the language over on the DVD). It's easier to follow if I've seen it before so I can pick up more of the language.