As for Alan Moore's apocalyptic ode to masked avengers, there's been a lot of hemming and hawing about changes made for the movie, with some of the grousing delivered by Moore himself. He even removed his name from the credits.
Alan Moore is a big baby, because this is a worthy adaptation of his work. It captures a significant amount of the novel's paranoiac essence while making changes to keep things filmable. It's a nice homage to his fine work.
That said, the "unfilmable" Watchmen is quite filmable, and director Zack Snyder (300) does mighty good by it. Maniacal fans of the graphic novel will discover that the film is extremely faithful to large swaths of the tome--and it's almost always faithful when dealing with the characters of Rorschach (Jackie Earle Haley) and Dr. Manhattan (Billy Crudup). The ending gets a significant alteration, and some subplots (such as the Black Freighter sequence) are gone, but this film feels quite faithful to the source material.
It takes place in an alternate 1985, where Richard Nixon is still president; the Soviet Union and the United States stand poised for nuclear war; and most superheroes have been banned from practicing their vigilante art. Edward Blake, aka The Comedian, is thrown out of his apartment window to his death, and Walter Kovacs, aka Rorschach, investigates. He deduces that "masks" are being targeted, and he takes it upon himself to warn some of his former crime-fighting partners.
Dr. Manhattan, a scientist transformed into a size-shifting blue man after a radioactive accident, now works for the government. His superpowers almost single-handedly won Vietnam for the United States, and his presence on our soil is a big reason why Russia has never launched its nukes.
Snyder chooses to go the CGI route with Dr. Manhattan, and it's a wise choice. Crudup is mostly a vocal presence (and an oddly soothing one at that), but we do see him before his transformation during a flashback sequence. That sequence, with Dr. Manhattan recalling his past while vacationing on Mars, is one of the film's best.
Other retired heroes include Nite Owl II and Silk Spectre II (Patrick Wilson and Malin Akerman), each successors of a previous generation of superheroes. Wilson, to me, offers an improvement over the novel's bland characterization, while Akerman is a letdown. She has the look of Silk Spectre II, but her characterization is surprisingly flat. She's not terrible, but noticeably less effective than those around her.
However, Haley is Rorschach. I felt the same way watching his adaptation of this character as I did while watching Anthony Hopkins as Hannibal the Cannibal in The Silence of the Lambs. Haley's coiled rage is always evident through Rorschach's constantly shifting inkblot mask, and it's downright frightening when that mask comes off. It's a great thing to see this man in movies again.
The changes to the ending, while a bit drastic, don't do anything to harm the effectiveness of the picture. Contrary to some reports, it does not slander Moore's original vision. The alien squid from the novel is replaced by something different, but the message remains the same.
Snyder and his cast are truly successful with Watchmen, a project that had a lot of stops and starts. It's a swirling, hallucinogenic, crazy adaptation of a nutty book, and I am more impressed with what Snyder managed to get into the film than I am disappointed by what he left out. It stands alongside Christopher Nolan's Batman films as one of the best in the superhero genre.