I have not played it directly, but it's a d20 game, so as far as mechanics go it's pretty simple. Being the gigantic nerd that I am, I've been listening to the rolemonkey's podcast where they're playing a serenity campaign. it sounds like they're enjoying it. http://harpingmonkey.com/index.php?option=com_content&task=category§ionid=6&id=28&Itemid=51
I'm a big firefly fan, if I had the time i would love to get into a group playing it, but alas..