Serenity offers a great time at the movies. I'd be very interested in knowing what non-Firefly people think of it, if any of them actually go see it, because some effort is made at the beginning to gently nudge newbies into understanding the basic setup of the Serenity universe and meeting the main characters, so it won't be obscure even if you've never seen Firefly. But I suspect the resonance the film will have with fans of the television series won't easily be transported to people who don't already love the characters.
While Joss Whedon has done some writing for films in the past, Serenity is really his movie coming-out party, the first time he's written and directed for the big screen. It's hard not to feel proud, watching Joss' work in a movie theater. There are a lot of people out there who love movies but don't have the time or inclination for television watching, and there are also plenty of television watchers who have managed to avoid Whedon's teevee work as well. So it's a joy to hear Joss' dialogue in a movie. The standard Whedon trick of having his heroes and heroines mouthing snappy patter in the face of danger is reminiscent of Rio Bravo. It's hard to explain just how wonderfully odd it was to hear Whedonesque dialogue coming out the mouths of movie characters.
As for the movie itself, I think it's better than the teevee series, but ... a movie offers plenty of pleasures unavailable to television, but the reverse is also true, and Whedon's television shows, most notably Buffy the Vampire Slayer, all focus on the strength we get from communities that are never stagnant, but are always vital to our well-being. Come to the characters of Serenity cold, and you'll like them enough, in an action-movie-hero kind of way. Come to them from Firefly, and you'll already know them, know how they interact with each other, and the characters will matter. They'll matter because you've seen them in a dozen or so episodes. I don't know that any single episode of Firefly is as good as Serenity, but the best compliment I can pay to Serenity is that it's like the best episode of Firefly. And I wouldn't blame moviegoers for wanting something more from a film.
Me, I'll just savor the dialogue, which, it should be added, is delivered perfectly. It's not really worth it to quote the best stuff, because what makes it "best" isn't just the words, it's the delivery. But the style is instantly recognizable to anyone who's ever watched an episode of Buffy. Eight on a scale of ten.
Finally, I've hopefully avoided spoilers in the above, but I'll add one note here. My least-favorite character in Firefly was River. But in Serenity, she wins the heart of this Buffy fan. Sarie, if you're reading this, at one point I half expected you to jump out into the theater yelling "BUFFY!" just like you used to back in the day.