Serenity RPG = Awesome. I have it, and I love it. It helps that I'm a browncoat.
From what I've read, it can be deadly, but it also has a lot of very good things going for it - relatively simple to learn, it encourages multiple types of play (very useful for a newbie game, in my opinion - basic D&D dungeon crawls were never just about combat, which I think a lot of people can forget), and is remarkably well researched. Plus it was written with newbie players in mind, it seems - which a lot of licensed properties do (since it's a safe bet that at least a few fans of the franchise will buy the book, having never played an RPG before).