I have seen Serenity. Sufficed to say that this film lived up to my expectations, and surpassed them. It did bring in the uninitiated whilst not repeating things for those familiar with Firefly.
It's a barnstormer of a film, and wraps up the series nicely, whilst at the same time leaves things open for more.
I kept myself spoiler free for this film, and if you know Firefly I suggest you do too. Sufficed to say that Joss Whedon caused my jaw to hit floor in a couple of places, there was one place in particular where I couldn't believe what had just happened (soon after an emergency landing) in the second half of the film.
I cannot emphasise strongly enough how good this film is.
See it in the cinema (you're too late - but it's now on DVD)
See it soon. You don't need to have seen Firefly, but chances are that you will want to afterwards!
Oh, and Kaylee fans should watch for a line soon after Mal checks his gun into storage as our guys enter a bar....