Both films are fun, exciting, and enjoyable. They pay decent homage to their schlocky forbearers with "Planet Terror" being the most well-rounded of the double feature. "Death Proof" is great, in parts, but Tarantino's dialogue for his female characters lacks the impact or snazz of Resevoir Dogs or even Kill Bill Vol. 2. Sure he's trying to make a B movie just like "They call her One Eye" or "Detroit 9000", but his characters, except for Kurt Russell as Stuntman Mike, are so boring, so thinly drawn that it seems like poor imitation of his earlier work.
Rodriguez, on the other hand creates some of the most fun and badass characters of his career especially with Dakota and Doc Block. His film is more rooted in the 80's gorefests that the Grindhouse era though. Not to mention Tarantino's cameo destroys Planet Terror for me, maybe that's the point though. But please Quentin, stop acting.
Bottom line; Zoe Bell is awesome as herself, and Death proof is fun with some awesome car chases. It's the ridiculous dialogue that spews from Tarantino's chatracters mouths that kills the film. Planet Terror is a gore fest romp that loses its way at the end. The best part of Grindhouse though, are the faux trailers, except for Rob Zombie's that is.