The creator of Buffy the Vampire Slayer (television series) also created a short-lived, if cult favorite, sci fi series entitled Firefly. He used the idea to make a small budget big screen film -- Serenity. It has also been a favorite, now for more people, including some on the blogosphere (Political Animal/Talking Points Memo) ... and one might add me to the list as well. As Kevin Drum (PA) notes, it is a good B-movie, not a masterpiece, but simply put, enjoyable entertainment.
I might add that the sci fi story of a rebel ship with a mysterious young passenger on board (a target for an assassin, played by someone I caught on a BBC lawyer series) also must be taken with a couple provisos. One, a couple plot points are of the "necessary for advancement of plot" variety, though since the movie does not take itself too series, this is not really a problem. And, second, for light entertainment, a part of the plot is pretty damn tragic. I am of the mind-set that certain tragic things need to be earned in films -- if the film is too lame, it does not warrant such downer plot developments. This film is well made (and acted -- with a light touch) enough to earn such things.
And, as with Buffy, there is an underlining seriousness to this film -- as with a vampire show, we are not to take it too seriously, sure, but it must be of some note. This adds to the enjoyment in the long run. [Buffy was a rather serious show, which is why it was a favorite a lot of smart nerd sorts along with others.]