I've just been to the cinema to see Serenity. Best! Film! Evar! Maybe. Go see it and judge for yourself. Joss Whedon has managed to achieve what people thought couldn't be done: a science fiction movie with a fantastic story as well as riveting action and special effects. That seems to have confused the hell out of the film critics, who just don't understand it. I read a review recently which only gave it three out of five and complained that it ripped off Star Wars and Blake's 7. Even if that was true (there are a few parallels) I fail to see how that is a bad thing. Star Wars had brilliant action and Blakes 7 had great stories. Serenity has both.
Beg, borrow, or steal the Firefly DVDs and watch them first though - you'll get a lot more out of Serenity if you already know the characters and the back story.