Holly and I saw Serenity on Tuesday night, and it was great. While I'm a moderate Joss Whedon fan, having watched Buffy and Angel, and most of Firefox, I'm not hard core, but I loved the movie. Holly didn't want to go at first, but after I promised to take her out for a steak dinner, she relented. She never saw any of the series that spawned the movie, but she was forced to admit she "really liked it" afterwards. I like science fiction books, but despise the majority of science fiction movies, mostly because they're bad. They tend to focus on the visual while neglecting the story, and are almost universally dumbed down to the lowest common denominator. For every Gattaca (which I loved) there are ten Red Planets (which I loathed).
In any case, go see this film. It's good, very good. Certainly the best Sci-Fi movie this year, as it focuses more on stories, and not on technobabble or dressing people in shiny pants.
As usual, here's Ebert's review.