It's almost slavishly faithful to the source material, for most of the film, treating the comic as both a script and a storyboard, for much of of the running time. Some sub-plots have been dropped, and that mostly ties in with the changed ending. The pirate comic's gone, too, about which I'm quite relieved. But even where the threads have been removed, Snyder's included passing nods.
The casting is mostly excellent, particularly Nite Owl and Rorschach - visually, most of the cast catch the appearance from the book quite well. NIte Owl's costume has been changed to be even closer to that of Batman, but that makes sense - his original looked quite silly, too.
Veidt's a bad casting, IMO. He's completely lacking in the charisma necessary, and he's physically not imposing enough, either. Or old enough: Veidt's supposed to be in his forties (Goode is thirty).
Beautiful opening titles, set to Dylan's Times they are a-changing. Music generally is quite well-chosen, with numerous effective segments.
Fight sequences were variable: when each of the key figures (Ozymandias, Comedian, Nite Owl, Silk Spectre, Rorschach) were fighting each other, it was a let-down. Too slow, and way too much punishment taken without taking damage (with the exception of Dr Manhatten, these are ordinary human beings, in masks; Manhatten is the only super-powered being in the entire story, which is partially the point). On the other hand, the fight sequences when Dan and Laurie get mugged and during the prison break - when the key figures are fighting stunt men, that is - it's much faster, much more effective.
There's an extra (and unnecessary) sex scene thrown in, alas. Moore just shows the lead-in and aftermath, while Snyder takes advantage of his R rating to get a bit of shagging in. Shame. On the plus side, he got away with lots of Dr Manhatten full-frontal nudity, and it's No Big Deal.
Rorschach's mask was fantastically well done. Impressed with that.
Overall, a very successful adaptation, making for a good film. I'm not convinced it's a great film, but still very enjoyable.