I watched it mainly as research for when I went to see Serenity earlier this afternoon. Seeing that episode, where I learned a few things about the characters and the "'Verse" that they live in, helped understand what was happening in the movie. Let me say if you just walk off the street and don't know anything about what has come before, you are going to be a bit confused by what's going on. The creators try to get in a few things that will make a newcomer comfortable in the "Serenity World," but I'm not so sure audiences of today are really that smart. Plus, the movie looked like a two hour episode of a television show. The special effects were outstanding (even featuring a battle scene that rivals the opening battle to Revenge of the Sith!), but some of the sets looked like they were done on sets leftover from the new Star Trek TV series'.
All in all, it wasn't too bad, tho'. If you've watched Firefly, you're probably going to like Serenity, but if you've never seen it before, you may want to pass on it.
EDIT: It should be interesting to see how it comes out against its only real competition: the Jessica Alba, Paul Walker vehicle, Into the Blue this weekend.