I think one of the most difficult choices to make is whether a great movie is worth buying on DVD or just renting for an evening viewing. When I suggest is to ask yourself if you can see watching the film over and over- will you want to show it to friends and family- or do you just want the deleted scenes and the commentary? For me, films like the James Bond collection are worth watching every 2-3 months. I love the music, the style and the ability to look away if I'm working but still know what scene is playing. I love the rhythm of those movies, and how I can sync my body clock to that. I also like to collect the "classics" like Hitchcock's "North by Northwest". Here's a movie that I want to own just to know that such a perfect, genre bending piece of cinema is on my shelf.