I saw Serenity last night at a special sneak peek. I am a fan of Whedon, though no apologist (his work fulfills and infuriates me simulatneously) I have never seen Firefly (hope to pick up the DVDs soon) as I wanted to wait and go in "cold" to this film. The version was not finalized, it had some temporary "placeholder" special effects and bits of score from other movies.
I have to say that the movie is, overall, a success. The strength is, as usual for Whedon's stuff, in the interaction between the characters. I'd never met these characters before last night and became enamored with them pretty quickly. The writing is (for the most part) efficient in developing their relationships quickly and then deepening them later. The setpieces are relatively hit or miss but one towards the beginning hits so well that it pretty much won me over for the rest of the movie.
As a Firefly virgin, I wish Whedon could've spent more time developing the political landscape of the universe he's created here. Some elements of the plot hinge on political ramifications of actions and I felt I was waaaaay behind the learning curve at certain points in the movie, being unsure as to why some things were as big a deal as the characters felt them.
I had a very good time watching this and can't wait to see the show now.
Any questions from Firefly fans? I'd be happy to talk more about it either here or in email.