I saw Serenity at the blogger's preview last night, and I was very pleasantly surprised.
My guess is that this will be the science fiction movie of the year, and one of the better movies of the year. If you don't hate science fiction, you should see it, whether or not you have seen the TV show.
A brief synopsis: In the far future, most of humanity lives in a star system with hundreds of terraformed and habitable moons and planets. (I know, but go with it.) The inner planets are ruled by the "Alliance," a civilizing but somewhat corrupt government, and the outer planets tend to be a little more anarchic, something like frontier towns in the American West. Captain Mal Reynolds was a soldier on the losing side of a civil wara against the Alliance, and now makes ends meet by taking various jobs as the Captain of the spaceship Firefly. The ship is always in constant danger of falling apart and/or killing the crew, but somehow, Mal has managed to collect one of the best crews in space, with various genius-level (at least in their specialties) members, including a pilot, a mechanic, a doctor, a psychic, a mercenary, and a prostitute with a heart of gold. (Hey, it's a Western - you've got to have a prostitute with a heart of gold!).
The movie does a nice job of setting up a plot that ties into the TV series but stands fine on its own. The major plot is the Alliance's attempt to recapture the psychic on Mal's crew and her brother, a former member of the Alliance's creme de la creme who threw away his career and fortune to save his sister from the experimental facility in which they were keeping her. Whedon sets up the characters well enough that you don't need to have seen the show to enjoy the movie, and he alternates very nicely between drama, pathos, and humor. His dialogue is great (although I could do with all the "I reckons" and "I cogitates" that apparently give the outlier characters Western verisimilitude) and he has a real sense for his character's emotional lives. The actors are all first rate, and it's an engaging, emotional plot.
The cinimatography, on the other hand, is functional but forgettable. I don't think you would miss much if you saw the movie on a DVD instead of in the theater, but I would still go to the theater, if I were you, so that you could see the movie now.