Watchmen is a difficult movie to categorize . As an adaption of an existing work, it is excellent. The attention to detail is amazing, the cast all fit well with their characters and an impressive amount of the plot is exactly the same. A for effort.
As 
a stand-alone work, Watchmen is less of a success. It is very, very long and filled with talky scenes whose purpose only becomes clear later and sometimes not even then. It's not that it is a terrible movie, just that a viewer not familiar with the source material might easily lose track of what is going on. So much of the plot hinges on the motivations of the characters - the book provides extensive back stories that the film cannot linger on, leaving the story feeling a little flat.
The production values are first rate, with excellent special effects. The actors are all look the part and for the most part are pretty good, except for guy playing Rorschach, who is excellent. Watchmen is an ensemble piece and the fact that none of the characters are played by major stars works to its advantage. The R rating is very well deserved, the film is quite gory in places and there is a lot of nudity. Watchmen may be the first mainstream film to have more male than female nudity, which I guess could be called some sort of landmark.
On the whole, I would recommend Watchman to anyone who enjoyed the book. I would still recommend it to others that enjoy the odd superhero film, but others will probably be bored.
Here endeth the review. The following section contains spoilers and crowing about how smart I am. Discerning readers may want to stop here - you have been warned.
I 
felt that Adrian was the character most changed from the book, and not in a good way. In the book he is smug and arrogant but slightly unsure of himself, whereas the movie makes him out to be more calculating. Adrian's big speeches were not well handled in the film. I was particularly disappointed that his "I did it 35 minutes ago!" line was not made more of a big deal - it seems like that should at least get a reaction from the others in the room. On the other hand, his plan is actually a lot better thought out in the movie. The giant squid from the book fits in with the comic-book theme but really makes no sense either in conception or execution. Replacing the squid while keeping the same general theme of the plot allows the film to jettison a couple of minor sub-plots and ties up the ending in a neater bow.
Rorschach was perfect, even down to the voice. It was good to see that the producers didn't flinch from making it clear that their main character (and one of the "good guys") is a violent psychopath. It was a shame that his awesome fridge-based ambush was nowhere to be seen.
The film did not manage to improve on Jon's character - for a nigh-omnisentient being he is remarkably dense. I know he is supposed to be beyond human understanding, but none of his rationalizations in the book make any sense, and they sound doubly silly in the film. Still an interesting character if you don't think about what he says too much.
Now for the crowing. A few months ago I wrote an essay on how I thought the then-upcoming Watchmen film might turn out. Lets see how I did:
- The attempted rape and the identity of Laurie's father. I was wrong about this, it is in the film and discussed in detail. It still doesn't really make much difference in the scheme of things, but it is included.
- The Black Freighter story-within-the-story is missing from the film, although I hear it was produced and will be available separately for completists.
- A lot of the back stories were dropped or shortened, particularly Adrian's monologues and the book excerpts
- The atomic blue scrotum was 100% present
- I was surprised that Laurie just smashed the Martian complex with her fists and not a bottle of Nostalgia like in the book, especially since the film had set up Nostalgia as a brand name in the first scene. I guess the director thought that metaphor was just a little too groan-worthy to include.
- Some commentators drew issue with my 9/11 analogy but it is made explicitly in the film, with the World Trade Center in the background of many shots and a quick look at the rebuilding of ground-zero during the coda. I think it fits in well with the themes of the novel.
Well I guess my attempt to second-guess the producers met with mixed success. I am just glad that some of my fears turned out to be unfounded.