The movie is very faithful to the book, but it didn't seem slavish. (They did change a major mechanism for a plot point at the end of the book, and honestly, I think this version is better.) Some subplots were removed, but their absence didn't seem to be problematic to me.
With the exception of Matthew Goode (as Ozymandias/Adrien Veidt), the acting ranged from decent to quite good. Patrick Wilson (as Dan Dreiberg/Nite Owl II) did a great job, as did Jeffrey Dean Morgan (as The Comedian), but the really standout performance was Jackie Earle Haley as Rorshach. I don't know what was going on with Matthew Goode -- he's a British actor with a solid resume, but he sounded like a demented Dutchman in the movie, which might have been intentional -- but whether on his part or the part of the director is hard to say. It wasn't a big problem, in any case.
The special effects are impressive without taking over the story (and mostly drawn straight from the book), and the pacing seemed fine to me. Some people thought it could have been cut back 20 minutes or so, but I wasn't bored at all -- and I usually am if something is slow.
I'd like to especially note the opening credit sequence, which is simply fabulous and almost worth the price of admission on its own. It was given a shout-out by Teresa Nielsen Hayden at Making Light, and rightly so. (Go read the story. The lawyers at Warner's are jerks, and the Warner's upper management are idiots.) You can find the opening credits here.