OK, I am in the fan club.
Actually seen Firefly and Serenity a while ago, just wanted to let it soak up. (Plus somehow didn't find the time for a proper homage. Bad manfred.)
It is a wonderful story, with great characters and too many funny quotes to choose from. Should get an Oscar for the dialogues only!
To give a Citadel ranking to the whole thing, it is a clear 5.0 for me. The movie seems to be a bit weaker than the series (but there is THE Message included that makes up for it), and the first few parts, as well as the last part of the series did not appeal to me as much (so only 4.0-4.5 for those). As a whole, however, it is an excellent story, a lovable series that contains so much humour, action and emotions that would dearly love to see something like that in a campaign, mine or anyone's.
As an aside, you can't help but imagine putting a character or two into an RPG - take Mr. Niska, your ideal employer for any group even slightly on the border of law. Don't you cross him, or... things will be not so solid with you. Or the Operative from the movie, another psycho and excellent opponent.
In the end, the strongest aspect is the emotional impact - it is a human story. It is also great how the most funny moments are combined with the most chilling, or sad moments. I have seen it whole two times in a row, and may see it another time again. It is worth it.
EDIT: Advice to anyone watching it - see the series first, then the movie. While the movie is fine on itself, it is much better if you know the characters and their background.