SPOILER ALERT- well kind of
Not to take anything away from maverick film directors, especially Rodriguez (the Posse admires his stand against hollywood by insisting Frank Miller share the directing credit on "Sin City") but the two-part film he shares with Tarantino could have been vastly improved, simply by fixing Tarantino's end of things. Kudos to them for trying something different but...
First and foremost, switch the two. There is nothing to be gained by playing Rodriguez' vastly superior zombie shootout 'Planet Terror' before Tarantinos' slow building 'Death Proof' except to raise our expectations higher than what can be delivered by the second film. Leaving the theatre there is a deflated feeling as you try and remember how great the first film was. This switch alone would be a logical improvement and build the viewing experience appropriately.
Second, cut the long dialog between two separate scenes of two separate unrelated groups of girls. These go on absolutely forever without adding anything to the story or foreshadowing anything that could happen. One scene is dull, but two scenes is excruciatingly dull! Tarantino should tighten and edit these way down before considering any DVD release.
Third, please don't allow Quentin Tarantino screentime as an actor! He's not pleasant to look at or listen to. Playing small bit parts in both films is more about leaving his ego on the screen than acting chops.
What could have been a modern day classic is spoiled. Talk of splitting them up for separate releases may be right on target.