I've seen a LOT of polarized responses to this movie from people who liked the book, and as near as I can figure, it's because of what could have been.
Case A: A Hollywood movie, with a happy ending and likable characters and a wisecracking kid/narrator, with a changed setting and a PG-13 rating.
Case B: A relatively faithful telling of the story, with mostly reasonable cuts, but some questionable decisions.
Case C: The movie that goes on in your head when you read the book, with only the cuts you would have made.
I was expecting A and got B, so I'm pretty happy with it overall. I know some people who expected C and were disappointed.
The Archie love scene, and really most of the fight scenes, were far too long. I feel like Zach Snyder needs to figure out that no one's ever going to make The Lobby Scene again and so he should stop trying. He also needs to learn that implied violence has much more impact than explicit. Many of the performances were lackluster, and weren't helped by the characterization time being cut in favor of people ducking and spinning in slow motion. Rorschach was brutal, but he still came across as a likable guy somehow. Why was Richard Nixon played by a puppet?
On the other hand, Dr. Manhattan was excellent. He gave the impression of being cold, but there were facial tics to let you know he still had humanity in him. You got to see how broken the superheroes' lives were. The world felt right (A CRT with dials on it in the lab! Pay phones! Giant plastic glasses!), and the tension over doomsday was palpable. The costumes were spot on. You could tell the director wanted the movie to be like the book, and I guess that's really the most important thing to me.
I'm certainly not going to see it again in theaters ($16 for a movie and a small soda? Really?) but I think it's worth owning on DVD.