I saw Serenity today. I enjoyed it a lot, but I also spent the last two days watching nearly every episode of Firefly, the TV show of which it is a continuation. The whole series is a weird mix of science fiction and western, and the exact percentage shifts from episode to episode. There are episodes where it's mostly a science fiction show that happens to have random Western trappings in some of the costumes and dialogue, and there are episodes that are for all intents and purposes westerns, but where you might see a space ship at the beginning or end. The movie's balance is almost entirely at the sci-fi end of the spectrum.
Now I should point out here that when I use science fiction to describe Firefly/Serenity, I use it in the sense of "being set in the future/space." The term science fiction can also have thematic implications, of positing ideas about how things will or might evolve in the future, and then exploring how people and societies react to these changes. Firefly/Serenity is not science fiction in this thematic sense; the only real idea in the series is pure television high concept ("What if, in the future, people totally acted like it were the old west?"). Of course, science-fiction-as-setting is pretty much all you can hope for in most film and television science fiction, and given that Firefly and Serenity do a pretty darn good job.
I get the impression that the film would be understandable for someone who hasn't seen the TV show, but obviously can't speak from experience. The TV show comes highly recommended, but watching the movie first might put a different spin on the experience. The show started with a set of characters and was gradually revealing their backstories and secrets. That got put to an abrupt halt when the show got cancelled. For the movie, they sort of lay all their cards on the table. A lot of stuff is revealed that's only hinted at in the show, and I worry that, if you approach it backwards, it might take some of the impact out of the TV show.
Still, though, the whole thing, series and movie, is highly recommended. Not mind-expanding fare, but very well executed and stylish fluff. Interesting characters, fun plots, snappy dialogue. I'd see the movie in the theater if you can; if plausible, see all or part of the TV show first, but don't let that get in the way of seeing the movie.
My current roommate here in London is extremely obsessed with the TV show and the new movie. She has the dvd set and has a Serenity poster decorating the wall above her bed. I thought it was just some quiet little film with a small cult following, but it seems to be getting reviewed everywhere. I will most likely end up going to see it when it comes here due to her obsession.
I think it's worth seeing. In fact, I spent most of my time in class today wishing that I were seeing it again rather than sitting in class. It's not going into my list of Great Movies, but it's purely entertaining in a way that most movies haven't been lately.
If you can, I'd recommend watching the DVDs first. The 2-hour first episode isn't the best, but it sets things up nicely and gives you a hint of the style. It picks up very nicely, though.