Watchmen is a beautiful, beast of a movie that takes some time to absorb. Even having read the graphic novel the movie is a bit overwhelming, but there is no denying that it is stunning, intelligent, and pretty reverential to the source work.
Set in an alternate 1985 America where tensions between the US and the Soviet Union are high, Watchmen is about a group of costumed heroes coming back together to investigate the murder of a colleague. There is a looming doomsday and a crisis of conscience for Dr. Manhattan, the one superhuman character that is now a weapon used by the United States.
The opening title sequence to the film is absolutely extraordinary! It plays to Bob Dylan's "The Times They Are A-Changin" and it really leaves you breathless. At times the use of music in the film is perfect, and at other times it is a hindrance. The visual effects and imagery of the film alternate between gorgeous and horrifically violent. The film also takes some liberties beyond the novel to make it even more gruesome.
There is also some definite drag in pace of the film and at times it sinks under its many story lines and multiple characters. Overall though it tells the story well and develops its many thought provoking themes.
Depending on how hardcore you are about the novel will determine how you feel about the ending. I thought the re-write was fitting and tied very nicely back into the story. Dare I say I even liked it better? I don't know, but I think it adds complexity to Dr. Manhattan's character. Basically, instead of Veidt orchestrating an alien invasion, he harnesses Dr. Manhattan's power to make it look like he nuked New York City. The Russians and the Americans begin peaceful negotiations because they now have a shared enemy in Dr. Manhattan.
Definitely flawed, but I can't imagine anyone doing a better job of adapting this to film. I give it 4 out of 5.