FINALLY got to go see Serenity. What can I say that hasn't been said before?
Is Joss Whedon a narrative and dialogue genius? Hell yes!
Is this film better than any of the Star Wars prequels? Without doubt.
Does it step past the Original trilogy as the best Sci Fi film of all time? Close one, but the fact it gets consideration says enough.
Can you tell it's based on a backstory? Yes you can.
Does the film deserve a motion picture sequel? No it doesn't.
Why? Because Joss Whedon works best in the longer form of the television series. This story deserves that sort of treatment. A second motion picture would be a disservice to a wonderful narrative vision.
If you haven't seen it yet, ignore waiting until you've seen Firefly first. Catch it on the big screen and THEN go watch the DVD of the series as a prequel.