I'll say first that I am reviewing the film and I have not read the comic.
Watchmen is like many things I like, a contradictory thing.
It is what I feared and expected it to be. An overhyped, overclever somewhat slow treatise on the gritty realism of such unrealistic superheroes that amounts to huge amounts of narm. There's a blue naked guy walking around who can travel through time, teleport and manipulate matter using some kind of telekinesis. There is literally no reason for the gritty realism treatment. All it does is highlight how fucked up the film is. (Though granted that may be the point, seeing as this is based off the comic which deconstructed superheroes).
It is also on the other hand, once the plot gets moving, a nice somewhat rompy deconstruction of the Superhero genre.
Rorsach and the OwlGuy (Quite ashamed I don't know his name since he's one of the two people I actually liked) are like what would happen if Batman went through a Star Trek teleporter and was split into his two identities.
Rorsach is Batman as the great detective, seeming like a gross parody of seventies, Gene Hunt, Dirty Harry and Jack Bauer. The ultimate dirty cop with no morals but a sense for justice, a supremely unlikeable guy.
OwlGuy is a badass normal human being, who just happens to dress up as an Owl to fight crime. You don't even get the sense that being a vigilante makes him a mad man like the Batman is frequently portrayed as in such gritty deconstructions because OwlGuy's not the first superhero so it's less 'I am become justice' than 'This is really cool and I get to help people'
Doctor Manhattan is a nice deconstruction of Superman in that I hate him because he's out of touch with reality but I think I'm supposed to hate him.
Laurie's cool and while I don't think the film needed the back story between her mother and the comedian, it was nice to see her portrayed as a main character, with her own important sub plot. Something fairly ironic considering Doctor Peter Coogan on The Major Spoilers Podcasts pointed out that by analysing the comics, Laurie isn't a main character or important.
Ozymandias is a nice anti-villain because ultimately he is no worse than Rorsach or any of the other supers, only instead of believing in some retarded sense of justice when the fate of the world's at stake he actually has the balls to try and save it, despite the costs.
As for the ending, well obviously I went into this film knowing how it was going to end and that Ozymandia was the villain, although I like the fact that the adaptation changed the ending slightly to one that makes more sense. It's a better deconstruction of the comics universe than I think even the comic managed.
In the DC Universe, the answer to the question 'Who watches the Watchmen?' is 'Superman' because Superman is one hero who could never go bad and even if he did Batman could probably take him eventually. (Though that bit's informed from the Dark Knight Returns, not that I've actually read that yet.)
In the Watchmen Universe the answer to that question, thanks to Ozymandias, is 'Doctor Manhattan' because the human race has corrupted him and made him see the world like Rorsach and the Comedian so firing nukes is not a good idea because if the world's going to blow itself to pieces, Dr Manhattan will give it a warning shot first.
Regarding the whole thing about Rorsach's journal and the truth getting out, I think that it doesn't matter because the point still stands that the supers were complicit in the deaths of millions and Dr Manhattan is still out there, equally complicit. A nuclear deterrent doesn't have to be fired once people understand the consequences of its firing. The ending of Watchmen is Hiroshima, a horrible dreadful thing that shouldn't have happened but one that eventually led to people knowing where the line was and that death on such scale could only lead to mutual annihilation. For a movie all about the cold war that is an appropriate way to end it.
Also I can now safely say that Bloodheat, This Book is full of Spider's and Thud are nice deconstructions of Watchmen and are all so much better. Thud deals with the question of 'Who watches the watchmen?' for the Discworld universe. This Book Is Full Of Spiders is all about an enemy that wants to create a humanity, like that in Watchmen and neatly deconstructs the ending. Bloodheat is just Watchmen for Doctor Who, you'll have to read it to find out how but suffice to say Ozymandias has nothing on the Doctor.
I don't think I'll read the comic after watching this, I daren't think what Watchmen would be like if Alan Moore had even more time for navel gazing.