One year of this thread and nobody's yet to say they didn't like Serenity. Allow me.
But first, to put things in perspective: I loved Firefly. A hell of a lot. Man, what a series. So don't go all thinking I'm just a hater who doesn't get the genius of this story, these characters.
And neither, I think, did I love it just so incredibly much that ANYTHING, short of a full second series, would be a let down.
I thought Serenity had a weaker plot, weaker dialogue, weaker characterisation than Firefly. Alone, it was a sub-average sci-fi flick made above average only by the history of the series and the knowledge of story, character etc. that we start the film with (assuming we've seen the series).
Alright, so I'm not really saying I didn't like it. It was pretty enjoyable and great stuff to see the Firefly crew back on the screen. It was just less than it could have been. And this Serenity love-in of a thread needed a devil's advocate.