Put simply, one of the better genre flicks to come out in a while. It has a story, rather than a series of set pieces that have been strung together. Characters grow and change. It's harsh, funny and clever. For the Firefly fans out there, if you love the series, you're likely to enjoy the film.
The only real problem it has is the info dump at the beginning. It means the film gets off to a slightly clumsy start, but given that probably 95-99% of the people who will go to see the thing have never watched Firefly, it's a necessary evil. And I've seen far clumsier info dumps.
There's a point in the film where the audience spontaneously applauded. It's a moment that anyone, including those who have never seen the series, can appreciate. Not many films manage that. It's one thing to get a laugh, cheers and clapping are rarer.
All up, well worth a watch. I'll be seeing it again at the cinema as soon as it's out.
PS If you've seen the film and enjoyed it, don't run around quoting the damned thing! You got to see the film fresh and got a good laugh out of the lines, let others have the same enjoyment. That goes for the plot also. Too many people can't resist trying to be smug and clever and drop subtle clues. NOTICE - You aren't clever and they aren't subtle. A fair percentage of people can and will extrapolate your info during the film and guess the surprise or twist.
And this is a film that deserves to be seen fresh.