Yeah, I did it the other way around and it was weird. Though I loved Serenity the first time I saw it, there are so many things that happen in it that make more sense if you've seen the episodes.
Not to take anything away from the movie, but I actually like the show better. Even though there are only a handful of episodes, it's one of the best shows I've ever seen. And I owe it all to 3DGladiators. If I'd never come here, I might not have ever even heard of Firefly (though I probably would've by now) and, even if I had, might not have tried it.
@Taranis: you've gotta watch the series in order to get a true appreciation for it. That's probably why it didn't last, Fox ran it in a FUBAR order.