I've talked about Firefly on this blog before, and I have a character, with several pages of backstory, I created for a Trad Games Firefly game posted on my website. So, it would be pretty easy to say that I'm a fairly hardcore browncoat. Even if I wasn't, I would have been damn pleased to see Serenity.
The core cast is the same as in the Television series, and the writing is the same caliber it always was. They updated the look a little, and Serenity isn't quite as dark as she was back in the Firefly days. The animation is even a little better.
It's hard for me to talk about Firefly/Serenity without sounding like a retarded fanboy. Basically, Firefly is the most original and entertaining Science Fiction to hit the media in a very long time. Even if you're not really into SciFi, Serenity's characters and events are intriguing enough to make for an excellent show. You owe it to yourself to see this movie.