Like many other book/movie adaptations, I feel that the attempt to stay rigidly true to the source material hurt the movie. When I read the book, I was into the character development, background, and side stories. But when I watched the movie, a lot of it felt plodding.
Watchmen (either version) is basically a three-page big reveal writ long. I felt the movie could have cut more.
The movie was very graphic, wasn’t afraid to be bloody or brutal. It worked well.
I felt the film’s Rorschach was more tangible and interesting than I remember the book’s being.
I also especially liked the opening credit sequence. It’s artistic style and method of conveying the alternate Earth’s important events in short scenes were very well done.