Austenblog writes some thoughts on it here.
My thoughts: I think Ms. Miller hits it by saying this,
"However, you can read "Pride and Prejudice and Zombies" pretending that you're in it for the zombies or the laughs while secretly relishing Austen's sublime romantic comedy. At least, that's how I read it -- not, that is, with any pretense, but, as I went along, finding myself increasingly skimming over the interjections of horror and combat to get back to the real story....The great advantage, the secret weapon of "Pride and Prejudice and Zombies," is that when you tire of the beheadings and the martial arts and the blood and guts, it's still "Pride and Prejudice." And you know that can't be bad."It's all about Jane, folks. All about Jane.