It was a good movie, but you will be able to understand, why it is believed to be un-filmable. It is just too dense too ínclude everything for the comic purists! Snyder did a good job in capturing a lot of frames from the GN, like he did with 300. Of course a lot of scenes could not be in the film. At 162 minutes it is almost too long for the average crowd, but but not long enough for everything a die hard fan wants to see. A directors cut will hopefully fill the gap for the last group.
There was a lot of love for details & production design, which most likely will make the DVD a must among friends. Fanboys will miss "the little stories within", like the news stall, the lesbian couple, the media coverage in the Gazette & The New Frontiersman, which brought the alternate reality of a USA in 85 to life. Another downside is, that I think you have to have a good knowlege about the cold war situation and American history (Killing of Woodward & Berstein, Nixon still president, running for a 5th term etc). The normal 20 year old will miss a lot of things, that are easier to understand, if you are ten or twenty years older. Combined with the sometimes difficult to follow storyline, this will be the cause for a below expectations revenue at the boxoffice IMO.
Now about the story adaption itself:
First of all, explaining the "Keen act" (outlawing the masked heroes) should have been done better, as it sets up a lot of the behaviour of the heroes.
The plot around Veidt differs the most, but was perhaps the better story alternative then the "alien squid invasion" from the GN. The character of Veidt in the movie was bad IMO, the actor not suited for the role (too young and too "Bad Bond Villainy"; you will understand when you saw it). His backstory was also not as much developed as it should be to understand his motives. Both Rorschach & Dreiberg where great.
I could not see the reason why Rorschach killed the child molester like he did in the movie and did not "let him die" the way it happend in the GN, as it makes the turningpoint in the Kovac/Rorschach personality more clear. The scene involving getting to Kovac in the prison by Big Figure & his gang, was acually better then in the GN (which really doesn't make a lot of sense).
One absolute "no go" for me was the scene in which Laurie & Dreiberg fought the "Top Knots" gang in the back alley. In the GN, they just beat up the gang, leaving the scene "sexually arroused" by the fact that they both were "heroes" again. In the movie they brutally killed at least half of the attackers, which sent the wrong message (as both Dreiberg & Laurie never killed anybody in their active years).
Malin Akerman is really hot, so I am not bothered with the love scene in front of the TV, which was missing the important part: Dreiberg can't "perform" as a "normal" person, only later, when he is in his costume again and he has a second hot sex scene in Archie.
Akerman gives a decent performance as Silk Specter II, although the troubled relationship with her mother (because she forced her daughter into becoming a superhero) isn't explained. That's why the ending is a little bit strange (but thank god, they didn't chose the dyed hair variant from the GN).
I wasn't that much impressed by Dr. Manhattan, but both the acting and his scenes were pretty good. Of course there is the "male part" issue, which didn't bother me at all. The fact that Dr. Manhattan doesn't bother with clothes, is part of the story: He just doesn't care about humans and what they think or feel. The Comedian did a good job, his character fleshed out, despide the low on screen time. I think they kept almost all his GN dialogue.