If you haven't seen the movie don't bother reading the rest of this review. I wish I'd never seen Serenity as it may have ruined Firefly for me. I really recommend that people who loved Firefly just stay away from Serenity. It's different from the show and I personally feel that it's maliciously insulting to the show and the people that liked it. It will affect the way you view the Firefly episodes and may damage their re-watchability.
SPOILERS START BELOW
A day after finishing the last episode I ran out and rented Serenity to get my final fix of the Firefly universe. I was utterly and completely disappointed and wish I'd left it in the video store.
The hokey camaraderie from the series was gone. The well-developed characters betrayed their nature repeatedly like they'd never been developed in the first place. The deaths of 2 crew members were inexcusably pointless. The warm photography and sets were gone.
I felt something was wrong for most of the movie, I just couldn't get into it and wasn't nearly as involved as I was in every single Firefly episode. The turning point was the absurd killing of Wash. That was when I realized how much I'd been disliking the movie.
The Firefly series went to considerable lengths to make sure the characters acted in accordance with their nature. Sure they 'grew' occasionally or surprised themselves but in general they stayed true to their personality. Joss Whedon threw all of that out the window in Serenity. The doctor is extremely protective of River but he lets her walk around in a bar as soon as they leave Serenity? They strap the bodies of Shepherd's dead friends to the outside of the spaceship? Jayne would have done that, probably Mal, maybe Zoe after some coaxing, but everyone else? Kaylee talking about sex with the doctor when they're all about to die? She freaks out anytime there's trouble but she has time for that? Mal taking River on the bank heist job? Come on!
Wash dying was when I realized that Joss Whedon just didn't care. He'd been done with Firefly for a while in my opinion. But he'd be screwing himself over if he abandoned his Firefly fans at that point and refused to make a movie when it was offered to him. So he made one and his contempt for his fans and their adulation was as clear as day. Serenity was a slap in the face to everyone that loved Firefly. I'm just glad I didn't get caught up in the movement to save Firefly or I'd have been stomping mad after Serenity. I can look at the whole thing objectively because it's over and done with. I only waited a day between Firefly and Serenity instead of years like most fans.