Just watched it. My god, what a load of generic hollywood sci-fi fluff. It may have been better if I had seen the Firefly series first, but considering that's written by Whedon as well, I think I will probably give it a skip.
Halfway through the movie, I was wondering whether it was meant to be tongue-in-cheek or taken seriously. It's like watching Buffy (in terms of humour mainly) but the vampires are 'reavers', which are meant to be some sort of super aggressive, drooling man-eating zombies yet they can fly their own ships.
Another example of what makes this film so... what's the word... I'm not sure, but here goes:
Good guy gets a message from (ex-?)-girlfriend, which he knows is a trap, yet goes anyway. Ok, fine. Good guy turns up. Girl finishes meditating/whatever, and says "I didn't think you would be stupid enough to show up", and lights a little stick, and when asked what she's doing, she says that she's saying a prayer for good guy. Then the bad guy turns up.
After some talk, good guy and bad guy start fighting. Good guy gets his ass kicked, but of course he doesn't have any long-lasting injuries yet, and at that exact moment, girl confirms that she didn't light an incense stick, but it's actually some sort of flashbang, and good guy and girl get away while bad guy is dazed.
Corny? Bad/thin script? For lack of a better word, I'll go with "crap".