Overall, I have to say I wasn't really impressed. I think the film makers were just trying to make a cool movie about cool comic-y stuff and really didn't have a strong grasp of the story, characters or themes of the original work. It felt like a lot of 'gee whiz' and not much attention paid to the story or, in many cases, the characters. I'll go over my personal litany.
Let's start with Rorschach. He shares the role of POV character with Dan Dreiberg (Night Owl II), and we see much of the story through his eyes. He's very earnest and uncompromising and, in his own way, trying to do good. He's very intelligent, talented, clever and resourceful. He's also a crackpot, more than a little un-hinged, cruel, violent, misogynistic (misanthropic, for that matter) and just generally not a good person. The movie presents him in a much more positive light than he deserves. It conveniently omits a lot of his crackpot and misogynistic comments and the movie's general treatment of violence really dulls the impact of the horrible things that Rorschach does to almost anyone he meets.
Since I'm on the subject of how the film treats violence, I'll just say I didn't like it one bit. The fight scenes were so over the top as to be almost comical. The gore and blood were played up at every opportunity, quickly desensitizing the viewer to anything but the most extreme violence. This treatment really dulled the impact of a number of scenes that were meant to show character. The Comedian's attempted rape of Silk Spectre seems almost like a playful romp. Y'know, just a few bruises and cuts. She wasn't hurt that badly. I mean, there were no bones sticking out of her at weird angles. She didn't have a knife driven through her neck sideways. Nothing major.
The scene where Dan and Laurie fight off a gang of thugs in an alley strikes me as the most egregious example. The fight opens right up with an image of a thug's bone getting broken so severely it almost pierces the skin. The two "heroes" murder most of their attackers in spectacular ways. The scene is transformed from a fight for their lives to a bizarre courtship moment between two video game caricatures.
I could go on for a long time dissecting the film scene by scene, but I'm sure others have done that far better than I ever could. I'll skip straight to my other most hated scene, after the alley fight. The final scene in which Ozymandias reveals his master plan, fends off the smaller-minded heroes only to be defeated himself my Dr. Manhattan's near-omnipotence. In the end, his plan was successful, but the cost was very, very high. Finally, even the man who would be a God is shown to doubt. To be human. "Did it work out in the end?" No, the actual God tells him, nothing ever ends. Or at least, that's how the comic book ended. The movie ends with Ozymandias' vindication. He was right. There can be no doubt or human frailty in a figure as great as him.
Not to say I hated every second of it. I'll give the movie props for a few things. First of all, the actual content of the grand plot to draw the nations of the world together was much better. The explosions mimicing Dr. Manhattan were much better than fake alien squids. Once again, however, the impact of it was reduced. In the final scene, you see a number of character who had screen time in the comic, but not the movie. They're terrified and they know their world is ending. But who cares, they're not even characters in the movie until that last second. The comic spends a lot of time developing the newspaper seller, the black kid who's his patron, the reluctantly lesbian cab driver Joey and so on. When they die, it has an impact on the reader. My God, these people were just living their lives. In the movie, it's just an explosion.
All in all, it was disappointing. There were a lot of good moments and good characters, but that all mostly gets muddled into a poor understanding of the story and characters, and the pointlessness of all the violence in the film. I can't recommend seeing this and I can only say I'm happy, in retrospect, to have waited until I could rent it.