Spoilers ahead so beware.
Overall I was pretty happy with the movie. I had some minor problems with it, but I like how Joe framed it (you ought to read what Joe had to say) as about as good a companion piece to the book as could probably be expected.
I'm really looking forward to Snyder's longer cut that he'll release later this year - not because I'm a geek who desperately needs to see the pieces left on the cutting room floor, but because I think there were a few integral emotional notes in the story that were skipped over that would make it a better film, once included.
Mainly what I think it was missing was the sense of humanity and doom that comes from the conversations at the news-stand. As the film stands now, every character with more than a couple of lines is (or was) a superhero (or is Richard Nixon, who I frankly could've used a little less of), and there's not enough connection to what, to who, they're trying to save the world for. The scenes at the news-stand in the comic offer more than just colorful commentary and a connection to the Black Freighter story; they really underline the way real people are feeling helpless in the world of the story.
There were two times I actually teared up during the film; the second was when Rorschach takes off his mask at the end, and the first was when New York City explodes and the newsstand vendor and his comic-reading friend hug each other just as the explosion comes at them... now the only reason I was tearing up at that is because I knew who those characters were because I'm familiar with the book - if we'd actually been introduced to some humans, to them, I think the devestation of that moment would've been more effective.
Still, speaking of the change to the ending, I really really liked it and thought it worked really well. Sure I love the damn squid too, but what works on the page would probably not have translated, and even beyond that I thought it was an effective, The Day The Earth Stood Still touch, making Dr. Manhattan the Angry God watching over us, ready to strike at any time... perhaps I'm a big ol' cynic, but I find people's fear of being reprimanded by incineration a more believable impetus to brotherly love and cooperation than I do as a reaction to a an unknown quantity they can't fathom driving them to Utopia. People tend to need the teacher standing over their shoulder with a ruler set to knuckle-snapping.
As for the performances, like everyone else I thought the main trio of Jackie Earle Haley, Patrick Wilson, and Billy Crudup were phenomonal. Haley was especially fantastic. And I'm gonna come out swinging for Malin Akerman - the problem she faced was her character was whittled down to nothing. In the book, Laurie's a bit.... shrewish, but they softened her character a lot in the screenplay. And then there's the not smoking issue, which Joe addressed well... and that was such a defining characteristic for that character, and so integral to showing both her indecision and the stress she's under... well they erased most of what made Laurie interesting. I wish they'd found some other habit if the studio was gonna be so stupid about the smoking, something to show her waffling nervousness, something to give the character a little more depth. Then Akerman would've had something more to play. But as it was, I don't think Akerman deserves the vitriol that's been aimed squarely at her; with what she had to work with I found her perfectly fine. As for Matthew Good as Ozymandias, I still do think he was miscast, and there was just such a physical disconnect with what the character ought to be that he had some trouble overcoming it. Again, I didn't think he was awful, but I did find him to be the weakest link. And finally, Carla Gugino rocks.
There were long stretches of the film where I really felt Snyder got it right. Where I sat there amazed that I was actually seeing Watchmen as a real moving picture, with the characters and stories I knew actually being played out in front of me. Like I said above, I did actually tear up a couple of times, and Snyder and his actors definitely did find an emotional resonance that I missed from certain things on the page. I don't want to say that Zach Snyder is "misunderstood" as a filmmaker because really he hasn't made a perfect or even a Great Movie yet (although I have thoroughly enjoyed all three of his films), but I do think that a lot of mainstream-ier critics (A.O Scott and Anthony Lane I'm talking to you) just don't get him. I especially scoff at Scott's total misinterpretation of the tone Snyder went for in the sex scene between Dan and Laurie in the Owl Ship - it is supposed to be cheesy and you're supposed to laugh at it. It's riffing on those through-gauzy-curtain sex scenes from the 80s (think Top Gun), when the movie is set. Have you lost your sense of humor someplace up your own asses, guys? Is that why your head is up there?
But that Snyder mostly harnessed this massive beast into something relatively effective (and again, I hold out hopes that the longer version will add some needed humanity), against very very stacked odds, is very impressive indeed.