Jeffrey Overstreet has an excellent discussion of Serenity. There are spoilers, of course, but, honestly, if you haven't seen it already, what have you been doing? One of the nice things about the ship and its crew, in both the series and the movie, is that it manages to be a metaphor for the human soul without being too obvious or preachy about it. Overstreet does a good job of bringing out the key moral theme of the movie, namely, that moral focus requires some sort of conviction. (He also doesn't flinch from pointing out flaws in the particular way in which Whedon cultivates this theme.) I do think he is a little hard on the visual style of the film, which, while not radically innovative or ground-breaking, and while a bit uneven, is respectable, particularly for a first attempt.
UPDATE: Also, check out Overstreet's I Heart Huckabees interview.