Serenity - First of all: Go see it. If you are already a fan of the show, chances are, you already have. If you aren't already a fan of the show, go see the movie anyway. If possible, spend the 14 hours necessary to watch all the episodes of the show first - you'll appreciate the film even more - but the two friends of ours who had never seen the show before we took them to the movie still loved it. If you like science fiction at all, you owe it to yourself to see this movie.
It's Han Solo done better than George Lucas ever managed, even back when Han still shot first. It's a better depiction of space as "the final frontier" than any of the Treks. And it's all done without resorting to having sound effects in space like everyone else since 2001 has felt the need to do (there is sound during the Big Climactic Space Battle, but their excuse there is that it takes place in the upper atmosphere of a planet, not in full vacuum).
I'm not even going to describe the plot, because ideally you should see the movie without having read any spoilers* first. I will, for that very reason, praise the trailers for the film: All the ones I've seen manage to give a sense of what the movie is like without actually giving away the entire story (as most trailers any more seem wont to do). It includes a bunch of humerous lines of dialogue, most of which occur within the first fifteen minutes of the actual film, thus leaving the rest of the movie to be discovered as you watch it.
A quick aside about the TV show: Prior to a few weeks ago, neither Brenda nor I had ever particularly gotten into Buffy the Vampire Slayer. We both thought the movie was kinda cute, and I had gathered that the general consensus was that the TV series was even better, but we'd never bothered to watch it.
As I wrote earlier, the Sci-Fi channel recently started showing Firefly, and I TiVo'd the first few episodes. On the strength of those, Brenda insisted I go out and purchase the DVD set so we could watch the rest of them without delay. On the strength of watching the rest of the episodes, we have since purchased all of the Buffy DVDs, and most of the Angel DVDs, and are working our way through them. Firefly is that good, that I immediately trusted that whatever Whedon had done in the earlier series had to be worth watching (and I haven't been disappointed). Serenity is of similar quality.
*Later on, after you've seen the film, come back here and follow this link if you want to see the inspirational story I suspect Joss Whedon had in mind when he wrote the line, "I am a leaf on the wind. Watch how I soar..." That version doesn't quite use exactly the same words, but the sentiment is certainly there, and it makes sense in context, looked at with hindsight.
And then go read Julan Sanchez's Chock-Full-O-Spoilers review in Reason for a discussion of some of the libertarian themes in the film, as well as references to Sartre and Camus. And this one for more such discussion, and links to even more.