Although most diners serve food 24 hours, keep in mind the fact that some specialize in breakfast, and may not even offer a lunch or dinner menu, even if they're open during those hours. If your heart's set on chili or a steak, make sure you check that they serve them before you go.
One thing most diners have going for them are their large portions. If you're looking for a way to save money, try sharing an entrée with a friend, or make sure you get a doggie-bag before you go home. Most diners won’t mind slipping you a to-go box before you leave.