They kept true to the comic. The only major change is at the end. The alien squid is replaced by a different threat that makes a lot more sense.
The performances were excellent, especially Jackie Earle Haley (Rorschach) and Patrick Wilson (Nite Owl). They did a great job of brining those characters to life.
My only complains is the pacing could have flowed better. The film needed a greater feeling of urgency, since it was built into the story. I also feel some scene should have felt more intense. At times it felt too clinical. But over all they did an amazing translating the comic to the screen.