Note: I watched this movie as a lover of the space western series Firefly, of which this film is an extension. I therefore will not separate myself from the series in writing this review by considering it a stand-alone film. If the movie interests you, then I recommend seeing the series first and then this movie.
Serenity is an amazing addition to an amazing series. It is different in mood and scope, but these changes, while perhaps not necessary, were better-suited for the feature film, I think. It is less witty and more action-oriented, more epic and less personal (for many characters, though not all). It's darker and fiercer; there is less room for jokes and smiles because the stakes are so high.The movie fleshes out the already complex captain Mal and sympathizes River more, while most other characters are relegated to the background and expanded on very slightly. The themes presented, on choice and sin and on love's power, sometimes seem heavy-handed because of direct dialogue about it, but are thankfully never shoved down the audience's throat through repetition. Because of the series, the characters have a lot of depth going into the movie, so when they are fighting, I am on the edge of my seat hoping they will not die. And the movie goes so far in how thrilling it is; at one point I almost slumped, having lost all hope, thinking everyone would die and the only way out was for this to all be a dream (which it thankfully was not).
The acting matched the acting in the series (excellent). The cinematography was very similar, although it seemed like it tried harder. There were some shots that were much better than anything I saw in the series though. The music, sad to say, didn't live up to my expectations because I love the theme song and was quite miffed at its exclusion. Watch the series. And then watch this movie. I think you'll be pleasantly surprised.
IMDb link: http://imdb.com/title/tt0379786/