I thought the system was okay, but Savage Worlds is much better for a Serenity-style game, IMO. Off the top of my head, I had the following issues with the system:
-Competency: While the characters in Firefly do have moments of incompetence, they are rarely awful at what they do best. Wash might suck at a lot of things, but he's a darned good pilot. Mal doesn't accidentally shoot his own people. Kaylee doesn't screw up when it comes to fixing the engines, and so on. The die + die mechanic of the skill system in Serenity gives such a wide variation of results that it is very probable that your character will have issues with stuff that they're supposed to be very good at.
-Plot Points: I like the idea of Plot Points, but the system seems very reliant on them. First of all, it seems that you're expected to use them quite often to avoid the competence problems mentioned above. That means that the GM has to give out a lot of them, which makes them less special than receiving them for good role-playing/clever ideas/the stuff that the book mentions as worthy of gaining extra Plot Points. Additionally, since they're tied to advancement, I found that players are very reluctant to use them -- it's sort of like spending XP in D&D. I would ahve preferred that Plot Points be fairly rare bennies rather than something that is necessary to keep the game running smoothly.
-The Book's Layout: I got my copy about a month after they hit the shelves, so this might have been fixed in later printings. However, the lack of an index or even a well-described table of contents is a huge problem when it comes to looking things up quickly during a game.
That said, the three sessions I ran with this system would have gone on to be a much better campaign if I hadn't made some key errors as a GM. However, having since looked over Savage Worlds, I think it's a much better system for a Serenity game. It does a lot of what the Serenity system tried to do, but without some of the bigger bugs.