It is taking every ounce of energy I have not to make an "I watched the Watchmen" pun. Every ounce.
SPOILERFUL REVIEW AHEAD
I first read the book several years ago, and I re-read it recently. It's an effective deconstruction of the superhero myth. I liked it the first time, though I didn't find it very affecting. The story is more intellectual than visceral, but it's not really as deep as people make it out to be. On my second reading, I found a lot more of the literary depth. I noticed more visual motifs, more layering of meaning, the parallels of the comic-within-the-comic Black Freighter story, details in the artwork that foreshadow things to come, that sort of thing.
The movie was very faithful to the plot of the book. They boldly kept in most of the details, making few changes just to please movie audiences. Dr. Manhattan, a glowing blue character with actual super powers, is drifting apart from humanity and feels no need to wear clothes. The movie actually shows him giving the full monty while successfully drawing as little attention to it as possible. Not an easy feat! Kudos to director Zack Snyder for not backing down, and kudos to the studio execs who allowed it! I mean, it wasn't really that big a deal, but it does give the movie a "this is a serious movie for grown-ups, and we mean business" feel.
The movie managed to cram in most of the important plot points, something was thought to be impossible. I read an interview with the director where he mentioned all the little one or two panel scenes in the book.... For a movie, they'd actually have to construct a set for all those one or two second scenes! And yet, they managed to do that, it seems. That impressed me.
Many fans of the book have complained about the changed ending, but the ending really is essentially identical in tone and meaning. It didn't bother me. I'm dubious about how well the plan would really work, but the final scene of the movie (taken directly from the book) shows that we are supposed ot have those doubts...
The overall production design of the movie was brilliant. Again, it perfectly captured the feel of the book. This graphic novel is somewhat unique in that there are several pages of prose, typically fake source text (from magazines, books, and such) between chapters. The movie did a good job of converting these into television talk shows and other montages. These are shots where the director couldn't crib from the book, and he did a great job with them. Some added bits, especially certain famous victims of The Comedian's assassination skills, border on brilliance!
The acting was good, too. A lot's been said about Billy Crudup's portrayal of the most CGI Dr. Manhattan, about Jackie Earle Haley's Rorschach, and about Jeffrey Dean Morgan's Comedian, but the performance that I loved most was Patrick Wilson's Nite Owl/Dan Dreiberg. He brought a level of empathy to the character beyond what was in the comic. People compare Nite Owl to Batman, but in a way he has more parallels to Superman. Dan Dreiberg is awkward and occasionally low on self-confidence like Clark Kent, but whereas Clark is an act, Dan is his real personality. Yet, once he puts on that suit, well, it turns him on in more ways than one! This is what Watchmen does best: Expose the underlying themes of the superhero genre that we're usually embarrassed of. Crime-fighting, wearing latex, it's a rush! :)
So I'm mostly saying good things about the movie, and yet, in the end, I felt like something was missing: that depth I noticed in the book on my second reading. Alan Moore never watches any movie adaptations of his books, and specifically of Watchmen he's said that he designed it as a comic book to explore the medium, and that this would not translate to film. Of that he was right; Watchmen translated the story and images to the screen, but it was not able to translate the motifs and depth quite as well. Obviously some things you do in a graphic novel you just can't do on film, but I guess I was hoping that Snyder would push his own medium a bit. He didn't.
But I don't really blame him. He fought enough battles as it was. He got the essential story told, with spectacular visuals, and if the price was that it had to be a little less artsy, I guess that's the best he could do. I applaud him for doing such a great job as it is. I don't think this will ever be considered one of the great movies because the underlying story is just a bit lacking in emotion, but I think it would've been a tall order to ask for a better adaptation.