"Serenity" does something that most films nowadays don't -- it works, it succeeds on its own terms. The characters are engaging, the dialog is slyly humorous, the interactions between the actors are unforced and natural. Perhaps shooting a season of hour-long episodes has taught the ensemble cast to work together in a way they wouldn't on a normal film shoot. It's not overstating the case to say that Serenity is the film that all the Star Wars and Star Trek movies wished they were.
The fact that Serenity got made and got distribution at all is down to what must have been a titanic effort by its creator Josh Whedon. How often does a film get made based on a television series that was canceled before finishing its first season? 'Serenity' has the look and feel of a labor of love, coupled with the talent and craft it takes to make a good movie. 'Serenity' isn't high art, but we don't live in a high art world; it is purely pop. It deserves to be popular, and you should support it by buying a ticket.
The reward for all of us for supporting 'Serenity' will be the possibility of a sequel. Sequels are usually shit, but since 'Serenity' and 'Firefly' are by definition a serial entertainment, a sequel would rule. I'd love it if Whedon made ten more.