So. Watchmen.
I must resort to metaphor.
Imagine a wonderful and complex painting, the size of a wall.
Painted on the wall, actually.
That is Watchmen, the 'graphic novel' cough comic book. Every bit relevant to every other bit, fractal trompe l'oeil hologram glorying in two dimensions. Scaling new heights of Flatland complexity.
Now imagine a bas-relief. The bas relief covers the painting, and is *exactly* the same scenes, rendered into 3D. With loving, fetishistic care.
Now imagine that only perhaps 2/3 of the painting is covered with this bas-relief. The rest is still flat.
Now remove the painting.
That is Watchmen the movie.
If you know the painting, it's a remarkable work of art, and even the decisions on what pieces *aren't* there, you can see, were made with art in mind and perhaps even pull it off. But in your head, it forever floats above the ghost of the whole. If you never saw the painting, you might still recognize that this is a work of art. It might still move you. You might even gain much of the original 'feel' of the painting. But all you'll know is that there are these inexplicable bits missing.
Others have ably reviewed the book as a book, and the movie as a movie. I'll leave that to them. I just wanted to offer my interpretation of the book-as-a-movie, and to note a completely non-sequitur observation no-one has mentioned yet. I really enjoyed the fact that the actors, and their characters, were all explicitly made up and acted as if they were being played by popular 1980s actors. Dan Dreiberg is Chevy Chase in the Fletch years, both visually (down to the glasses and the chin dimple) and behaviorally (simpering smiles). Wally Weaver is Woody Allen, including accent, voice tones, glasses, hair, stature, schnozz. And Walter Kovacs - Rorschach without his face - is Clint Eastwood of the Dirty Harry years, down to the square angular features and the trademark snarl, to say nothing of playing the violent stranger at the fringes. The comparisons of Adrian Veidt to Steve Jobs have been well-hashed over elsewhere.
Sally Jupiter looked a bit like a Susan Sarandon or a Holly Hunter. Not sure which. But that's as far as I go.
As I am a fan (in all the senses of the word) of the comic, I really enjoyed the film - not so much as a 'good movie' but because it was a herculean piece of fan service. I could feel Zach Snyder swearing to himself that no matter what the box office take ended up being, he was damn well going to film a three-dimensional version of the graphic novel because it (as a thing) deserved no less.
I liked it.
One question I do have, however, is this. Given that the schlong Dr. Manhattan has hangin' out the entire comic book is quite modestly sized, why did they feel the need to CGI Billy Crudup's junk to Boogie Nights proportions?