Actually, that is going to be a given, because there are two very distinct audiences for Serenity, those already familiar with the characters, and those who are not. For those familiar, you are going to love this film.
For those of you who are brand new to this 'verse, don't blink during the first fifteen minutes of the film. There is a lot going on in giving the audience a crash course in the personalities of the characters (the panning shot following the captain through the ship was very well executed).
The best parts of the movie: Joss Whedon's writing is right where his fans expect it to be. There is a great deal of humor in the character relationships, even in the tensest moments. The Operative (played by CHIWETEL EJIOFOR) was pitch perfect as the soul-less face of the Alliance. No emotion, no self, only the role granted him in the greater machine. No sneering, no cackling, yet this guy is just flat out scary.
Down side: While Joss Whedon uses every bit of space the big screen gives him, I think the medium restricted him too much on time. There was too much that needed to be done, and I would have prefered to have had more character time than special effects budget.
For fans of Firely, this is a must see. For a fan of science fiction, you won't be disappointed. For me, it was good to see some old friends again.
Doc in the Box offers his assessment.