Serenity is currently 80% fresh on Rotten Tomatoes, with a Cream of the Crop rating at 89%. Not bad for a mid-budget movie based on a TV show that didn't even air a dozen episodes.
If you've seen Firefly, Serenity will absolutely blow you away. If you haven't seen Firefly, Serenity might still blow you away - but even if it doesn't, it will still be one heck of a ride. If you love witty dialogue and clever wordplay, you will drool all over this film. If you like watching characters interact more than you like watching slick CGI ships blowing each other up, you'll love this film. (Although as a bonus, you get to watch slick CGI ships blowing each other up, too) If you like seeing really pretty people (and I know from first-hand experience that Nathan Fillion is indeed very, very pretty) on a very big screen, you'll love this film.
I do have two warnings:
1) Do not read any reviews about this movie until after you have seen it. The less you know going in, the more you get.
2) This movie is very intense. I wouldn't recommend anyone under 11 or so watching it. There's no gore, no sex (although there are a couple of quick sexual references in the dialogue) and most of the non-martial-arts type violence is offscreen, but this movie grabs you from the first ten minutes and doesn't let go.
This weekend, get thee to the movies. And find Serenity now.