This is like the best ever episode of Firefly - funny and clever with shocks, thrills and tears along the way. The cast are all great and Chiwetel Ejiofor does a fantastic job as the evangelical murderous bad-guy, The Operative. And, finally, two of the big mysteries are kind of solved - what's with River and what about the Reavers?
Even if you don't know what any of that means, Serenity's well worth seeing, and can be appreciated just as you can enjoy a Star Trek movie without knowing Spock from Pickard. The Serenity universe is much more human than the Star Trek one though, and full of the incredible detail and human drama that Joss Whedon does so brilliantly. As he says on the extras, these people are the ones that The Enterprise would pass by at warp speed without noticing.
So, I really recommend it. However, if you think you're going to love this space-western of heroic bandits, evil government and messed-up telepathic fighting-machine teenage girls, then start with the Firefly series, as watching this first might give away a bit too much.