The prequels sucked except for Revenge of the Sith, which was pretty good but not up to the standard of the original three films (the first two in particular; I could have done without so much of the Ewok puppet show in Jedi and to be honest, it was a pretty big hint of what was to come with Jar-Jar Binks in The Phantom Menace).
But yeah, I'd say to watch at least the original trilogy. They're probably not as great as some people say they are but they're still an integral part of film history and a lot of popular culture references won't make as much sense if you haven't seen them.