One of the best times I've had in a movie theater with all my clothes on, I saw Planet Terror as part of Grindhouse in an empty theater with two of my friends. We were able to react loudly and rudely, which is kind of what the movie demands.
Planet Terror was Robert Rodriguez's contribution to Grindhouse (a double movie with Quentin Tarantino and a bunch of talented directors contributing trailers). It is about an experimental gas that turns people into melting zombies. The fun is in watching the big cast get narrowed down. Jeff Fahey, Michael Parks, Josh Brolin, Bruce Willis, and especially Rose McGowan chew up their lines and spit them out like juicy tobacco. Parks plays his cop character from several Rodriguez/Tarantino movies (like From Dusk til Dawn and Kill Bill) trying to survive as his family turns to zombies. Josh Brolin is a murderous doctor trying to keep his wife from leaving. The show is stolen by McGowan as the stripper who loses her leg and replaces it with a machine gun.
The tone is goofy but gory throughout. With characters like the crazy babysitter twins, you aren't watching high art. Naveen Andrews (from Lost) plays a guy who cuts off people's testicles and keeps them. There are lots of "aw, hell no" moments where you see things you wish you didn't but it is all in good fun. A few moments threaten to cross into straight up awesome (Freddy Rodriguez doing a knife fight in a hospital is one) but it mostly stays true to the grindhouse premise of b-movie fun.
If you aren't shy about bodily fluids, needles, gore and ooze, and you enjoy a solid fun movie, check it out.