I watched Serenity today, and I thought it was an excellent sci-fi adventure. I don't think it's necessary at all to have seen an episode of Firefly to enjoy it; the characters are introduced quite well. Having seen the series beforehand certainly gives you another angle to appreciate the plot, but since the entire movie centers around River, and she's pretty much a mystery throughout the TV series anyway, it doesn't really give you an upper hand. As an added bonus, if you've seen the show, the movie also reveals something that you probably didn't even realize was a mystery to be revealed.
I found the hand-to-hand fighting, space combat, and hovercraft chase scenes quite amusing. The movie's funny, just like the show, and if the western aspect of the TV show bothered you, you'll be happy to know that it's essentially eliminated, providing nothing more than a slight flavor to the setting.
I also saw The 40-Year-Old Virgin. (Today was movie day.) Not nearly as entertaining. Extremely vulgar and crude. Normally I'd expect that from a movie whose title might as well include the word "sex," but I had heard that it was different. And, in a way, it was. It was this weird mix of smart, cute, and incredibly vulgar. I was certainly amused, and there were some absolutely hilarious moments, but the elderly Indian man who loves that F-word got old reeeeally quickly. Full of plenty of Steve Carell goodness; expect little more than American Pie for Grown-Ups and you'll be happy.