[Guest blogger Scott revises some of the comments on Death Proof around here in his slightly longer argument about Grindhouse, and makes an interesting conclusion.]
First of all, I maintain that Grindhouse is a single film or at least a singular entity, bottom line being that both films must be viewed in the context of the other to be fully appreciated. This is especially true of Death Proof, which I will explain shortly.
Planet Terror is the least complex of the two and, as a result, the one most capable of surviving on its own, however, without Death Proof as its companion it is merely a fun homage to grind house films of the past (It also makes sense for Rodriguez to do the straight-up homage because he's more of a chameleon whereas Tarantino is much more of an auteur).
Death Proof is the more complex of the two films. Not only is it stronger on a technical level in terms of the actual filmaking; it is far more subtle. This isn't a recreation of grind house films like Planet Terror it is a deconstruction/revision of them (specifically the slasher, revenge, and car chase genres). This is why it relies on Planet Terror. If I say to the average movie go-er "Death Proof" is a revisionary narrative of grind house films" Their response would, most likely, be "Great! What are grind house films?" Few people today would have any idea what I'm talking about. Pop Culture Junkie that I am, even I don't know first hand what grind house is.
This is why Planet Terror (and, in my opinion, the trailers) are necessary. It acclimates the un-ininducted into the experience. Then, with Death Proof, something strange happens: The film begins to evolve. It's no secret that both Tarantino and Rodriguez grew up and drew inspiration from this kind of movie. The thing is, they took what they learned from these movies which were, let's face it, fairly disposable pieces of cheap, exploitative entertainment, not good for much more than a few laughs, and , in their own work, transform it into art (arguably, Tarantino is much better at this than Rodriguez). So, as Death Proof begins we are introduced to typical stock grind house characters only with greater depth and better dialogue (something Tarantino is famous for) but, still they remain characters rather than being real people. Jungle Julia and her crew are the typical victims in slasher flicks: they are 'doing bad things'; drinking, smoking pot, hooking up with/teasing guys. In other words, they're just asking for it.
Then, the change over, the film loses the faux aging and we're introduced to the second group of girls (by the way, the black and white segment in the extended edition adds nothing to the film other than another opportunity for Tarantino to display his foot fetish). This group is a departure, they are completely 'real' characters. It has been pointed out that they're all movie people but, so what, to Tarantino movie people are the real people in his day to day life. In fact, these might be the most believable, realistic characters that Tarantino has created. So, in the second half, he takes these 'real' people and puts them in a grind house situation. Still, take out the false scratches and such, the film can function on its own. Most people would still see it and 'get it'
The point that the film again changes is after the first leg of the car chase (which, by the way totally kicks ass and the use of traditional stunting and Zoe Bell as the star add a tension rarely seen in modern film making). Specifically, things go awry when, after Zoe emerges from the bushes unscathed, the point where any normal, rational person would say "Oh, Shit... we gotta call the cops!" a perky Zoe says "Phew, that was a close one... Let's go get 'im"
At this point, these 'real' characters begin a regression into the revenge seeking women of grind house cinema. Still, I'm buying it... I even buy the one chick shooting him. That's a plausible reaction. Where it falls apart (at least without the context provided by Planet Terror). Is the final few seconds where they drag Stuntman Mike from his car, knock the shit out of him and, ultimately, crush his head in.
So, these 'real', 'normal' characters have suddenly become cold blooded killers? It is also worth noting that during this sequence Abernathy's skirt, worn at a respectable knee length for the most of the film, is now worn at an exploitative point high up on her thigh. It is an, admittedly, cheap ending but it is a grind house ending. Death Proof has now taken us full circle back to the cheap thrill of Planet Terror. When viewed on its own, this ending makes no sense. However, as part of the larger Grindhouse film... project... whatever you want to call it. It makes perfect sense. In fact, in order for it to be a true grind house film. That's how it has to end.
So, in short... Planet Terror is a fun recreation of the grind house experience whereas Death Proof really makes you think about the genre (and all its various subgenres). Both films are great but I still see it as one big movie.