I'm playing in Serenity now, and to be honest I don't see what's in it that makes it feel like Firefly. The book is beautiful, the fluff and background material are great and its a nice read. But the rules feel clunky, none of us are overly happy with them, including the GM. It might just be a taste thing, that everyone finds changing what kind of dice you roll every time to be nonintuitive. Its not that its bad really, just that you could do about as well with GURPS or d20 modern or whatever. There are some good things in there rules-wise, the techno-babble is sweet, and the guide to chinese curses are a favorite.
We're contemplating some changes, like getting rid of the restriction that you have to declare all your actions at the start of the round.
The best licensed system I've used is Buffy. The Drama Points answer the biggest fundamental problem of that show, and to be honest many like it. How do you let Xander hang out with Buffy and not get squashed in two seconds? The drama points achieve this wonderfully. Yes, white hats get a lot of them. That means they can survive the occasional swipe at them, and they can do something cool every once in a while. But they are no substitute for a hero. For the non Buffy fans out there, this is like having Sam and Frodo in an adventuring party with Legolas and Aragorn and everyone having fun while still making everyone feel like an everyman/great hero respectively.
The setting works wonderfully as well. I think this is a key to how well a licenesed setting works. Buffy had that whole 'one girl in all the world' thing going on, but after season 7 that was out, and the assumption that there are lots of slayers and hellmouths and this game is about yours works just fine. The concept of the Hellmouth that explained why there were vampires and demons in Sunnydale every week work just as well in an RPG.
One of the secrets I think to running a successful licensed game is to set the game in that universe and have the characters do stuff like the beloved movie, but keep the actual main characters out of it. Maybe the occasional cameo, but that's it. I think Star Wars works because the universe is big enough to contain lots of exciting, important stuff other than what happened in the movies. So while in a Rebellion star wars game it might be cool to have the PCs refueling the Tantive IV from their secret base just before it gets captured, or see a transmission from Han Solo just before the assault on Endor while they are going after an Imperial arms factory many light years away, you don't want to put them *IN* the middle of the story. It might sound cool to have the PCs at the Battle of Yavin, but what exactly are they going to do? Polish Luke's X-Wing before he blows up the Death Star?
A licensed setting is about playing on an existing stage. The story should be about the PCs.
