The book was excellent.
The transition to the movie lost a good bit of the depth of the characters as well as diluting some of the themes of the comic. And to top it off, most of the performances were pretty flat. I also hated the change to the end of the story.
I think Zack Snyder is good at bringing a certain stylistic quality and feel to a movie. But based on 300, Dawn of the Dead, and now Watchmen, he doesn't really bring anything else to the table.