Joss Whedon must read this blog for advice, because he did not in fact fuck up that movie last night.
In short, Serenity is just thrilling from end to end, in scale and pace almost nothing like the contemplative character-driven TV series on which it's based (weird to call Firefly 'contemplative' when it's a fairly rowdy Western, but that's probably the biggest revelation in the film for me). The language has real poetic grace, and is among the most finely-tuned that Joss has written; the action is well-directed and weighty, pointing up how hollow so much screen violence/action is. And the performances left me longing to see the show again, to revel in the supernatural chemistry of Firefly's large ensemble, which (never mind the very personal attachment so many people have to Buffy et al.) was surely Whedon's finest.
To address briefly the Doc's point, from the previous Joss post: The manic genre-mashup style of Serenity only seems jokey. Sure, Joss has thrown in references to an armful of movies (and with a single sequence - 'I am unarmed.' 'Good.' - has cast his vote decisively, hilariously, and justly for 'Greedo didn't shoot first'), but he can go as deep as you like into the genre homage stuff. Think of that aspect of the film as a trailer for his more sustained work, and remember that he wrote, scored, and directed an hour-long movie musical, at least partly about movie musicals yet peering more deeply into his characters' inner lives than anything he'd written before, which could serve as a textbook on humanist appropriation of genre material.
It's almost not worth geeking out about the acting, the writing, the set-ups, the richness of the (implied) universe, the constant insight into character even amid the most outlandish fantasy chaos, because everyone who's ever seen a Joss Whedon creation knows to expect these things. Line to line, it's as well-wrought as any movie you're likely see this year. (And there are subtleties of cinematography and editing that shocked me, even as I found myself missing the mannered formal frameworks of episodes like 'Objects in Space', which clearly liberate the perverse, risk-taking side of Joss's creativity in ways not usually seen in his crowd-pleasing populist stuff - like the Buffy finale.)
But. Oh, but.
What leaves me unsatisfied is that it's over - the central mystery of Firefly was revealed, tying off two years worth of plot in less than two hours. As heartwarming and exciting as it was to see the characters bigger than life and doing epic-hero-type things out in the farthest reaches of space, and as absorbing as I imagine the film is for non-fans (imagine Star Wars, the original, if Han Solo not only held center stage but did a draft of the script himself), the huge events of Serenity deserved more time, more space, more build-up, more contemplation. I know this is going to sound wonky, but perhaps I can explain later: what happens in Serenity is too big for film. It deserves to be on TV.
(And the events that will shake Firefly fans most seem rushed, as a result - but for 'mere' filmgoers, is the pacing alright? I'm not sure I could say. I wish I'd had more time to breathe between portents and explosive set-pieces, and I wish the villain had been more of an apparatchik and less of a sui generis monster like the show's Jubal Early - the bureaucracy of the Alliance was totally compelling on the small screen, and some of the epic feel of the story was sacrificed to bring The Operative in from near left field. As the by-now old joke goes: well, maybe someone will buy the right to make a TV show out of it.)
That said: little Kaylee's line about batteries - you'll know the one - is worth the ticket price all on its own.
Here's to Whedon's band of Big Damn Heroes, done good service by their creator. If this is how it ends, it has ended well, I think, and that is more than fans would have hoped. So that's alright.