Summary: if you want to have a conversation in a bar with an SF fan in the next month or so, you must see this movie.
Otherwise, it does have good points. Much of the dialogue is sharp; the sets are clever and there's plenty of spectacle. It's pretty entertaining. You will enjoy your two hours.
OK, so what's wrong. Top of the list: this isn't a movie; it's a 2-hour TV movie at best, and it feels a bit padded for that, to be honest. Bugger all character development, unless you count minor characters dying and one fairly unexplained miracle cure; you feel as if you've been dropped into a series at the two-episode season finale, with plenty left for next season.
Basic plot: bunch of renegades left over from the losing side of a war find themselves on the run from the Evil Empire. They bust out the sister of one of them, who has been 'enhanced' by EE scientists. She's now a mad, cute, psychic martial artist, and she has the guilty secret of the EE buried in her brain due to a piece of brainlessness that happens prior to the start of the film. So the EE want her back. EE's a pretty nasty bunch, but the captain of our renegade band has a heart of gold (stop me if you've heard this before), and doesn't airlock the girl at this point. So what do the EE do to find her? Deliberately trigger her behavioural conditioning. Brilliant; she now remembers something critical. The crew decide to find out what's going on despite the fact it's incredibly dangerous. They find out what's going on, and oh, yes, it is the sort of thing EE would want to keep secret (though frankly if that's their only guilty secret then they're not really very far up the EE league, because it was an accident), though a moment's thought will convince you that it's not the sort of thing that could possibly happen in the way described. Luckily there's a nice friendly infodump (think Princess Leia) waiting for them, which EE haven't destroyed. So the crew grabs it; thank heaven for common data standards because this is one, and then, after various shenanigans, tell everybody much like the parrot in a Child ballad. Once the secret is out, EE have the chance to turn crew into strawberry jam, and let them go instead. The crew mend the ship and set off on another adventure.
You will have realised that we are in an Idiot Plot. The plot of this movie only holds together because the EE and their representative consistently act like idiots, even when they deny it. Every so often, captain-with-a-heart-of-gold also acts like an idiot, but this is normally explained away as being a function of the heart-of-gold. It's also an action movie, with all that implies. This comes as a bit of a shock to those of us who've been watching Battlestar Galactica; this is a different sort of future world, one where people get stabbed in the gut and carry on fighting. That sort of thing.
So, here we are, it's a two hour tv movie with a stupid plot and lots of pretty action scenes. Is there anything else wrong? Well, there's a thin line between homage and theft, and this film, over and over again, is on the wrong side of it. Whedon steals content shamelessly from other sources in pursuit of his story. I am convinced there is a drinking game here. Once again, I think this is the fact that it's basically a TV show. If you're just doing one tv episode, nobody minds that your mindless evil zombie cannibals are exactly like everyone else's, or that your science fiction cities come in shiny utopia and Bladerunner varieties. If you're making a movie, you're normally expected to do a little bit more work.
No cliche is left unuttered. One example: the crew turn up at scene of massacre in their former haven (which is imaginitively called "Haven") to find Only Named Character Present is now the only person left alive, captain-with-heart-of-gold gets to tell him he's not going to die just in time for him to say his last words. Another: the crew are facing certain death. So one turns to another and explains they regret they didn't get it together, and the other say oh, now I've got something to live for.
If you have seen plenty of SF movies and read plenty of SF books, not one moment of this film will surprise you. All the major plot twists are well-telegraphed, normally so well that you would have to actually be mentally defective to not work out what was going to happen. Example of subtle foreshadowing dialogue: "so, you mean we have to outrun both Space Fleet A and opposing Space Fleet B?"
Of course, none of this matters if we're watching a cheesy SF action-adventure movie. But out there in cyberspace, half my friends' list are talking about how great it is, how philosophically deep, what a masterpiece. Now, you know, there are films that sort of get to grips with the interesting philosophical questions of SF, and there's even some TV that does. This doesn't, and if you think it does, perhaps you should be reading more books.