Let me start off by saying, watch the show before watching this film, as it will increase your enjoyment ten fold. Not to say it doesn't stand alone as a Sci-Fi masterpiece, because it does!
Like many people, I am a huge fan of the original TV show "Firefly", which follows Malcolm Reynolds, and his crew aboard the ship Serenity. This picks up where the show (prematurely) ended. Ties up a few loose ends, and provides us with an incredible viewing experience.
One of the biggest strengths of the series, was the great character development, you really cared about all of them, even the less likeable ones! This film has everything, suspense, awesome action, very sad moments, the great humour that the show was known for. Having said that, it's much darker than the show was, but definitely not worse off for it! I was on the edge of my seat practically the whole time...
I won't give anything away, as I would hate to have had this ruined for me before watching. This is straight into my top films, Joss Whedon has proved what a great director he is, and it's criminal that this show was cancelled, however Serenity is a fitting end, and legacy, to one of the best shows of the last 20 years.
Buy it, watch it, love it.