I split the audience for this film into three groups: fanantics, fans, and virgins. I'm in the middle camp as I've read the graphic novel a couple times, but I'm not a comic book geek and don't worship the book. It is an excellent and amazing book, but a large part of that is the way it breaks the mold of traditional superhero novels. It makes sense that true comic books who are familiar with everything else out there would naturally think Watchmen is the greatest ever, and those people may or may not like this film. It's an amazingly tough job making a comic book into a film, as you not only have all the normal limitations of filming a novel (abridgement, pacing, etc.) but you also have to get the look and style correct to match the existing visual work. As a fan of the graphic novel, I thought they did a fantastic job. I can't speak for the fantatic or for someone who has never read the novel, but I thought this film was excellent. The look is awesome, the casting superb (I'd had some doubts going in but it worked), it struck me as amazingly faithful to the source material, and I was thoroughly entertained. I'd even go so far as to say that in some ways I liked it better than the book. The book is dense and complex, and while I love that, there isn't always time to absorb so much material. Also, the way the book is written, almost like episodes of Lost, where time is flexible and we jump between the past and present and future, it's so much to fit together that even though I've read the book a couple times, I have trouble remembering everything. I loved that the movie quickly brought back so many wonderful memories. Throughout the film I kept being surprised at scenes thinking, "I don't remember... oh wait, yeah, I do -- this part is awesome!" I love that now there's a version of Watchmen I can enjoy in a three-hour period. Granted, film is different from the graphic novel, and they each provide a unique experience, but for people who don't have the many hours needed to read the novel, the movie's a great way to get a taste of novel.
While I'm sure some fanatics will bemoan a missing scene or two, I was impressed at how much made it into the film, and at how clearly the story was told. The flashbacks weren't confusing but came at a logical time and gave you quick insights into each of the main characters without delving too much on unimportant details. The conclusion is terrific, with one of the most morally ambiguous conclusions ever, and I was relieved they didn't try to clean it up for the film by giving it a Hollywood ending. Hopefully it will provoke many to discuss things.
It's hard to say now how the film will live up in time. I think it will weaken in some ways -- there's so much dense backstory and setup in this epic work that it's tough to have a story that keeps moving ahead -- but I honestly can't think how they could have done this better.