This movie is polarizing reviewers. Artistic masterpiece versus incredible waste of time. I've been waiting for the movie for months, maybe a year even. And now, having sat through two and a half hours of it? I think it's better to invest a few hours into reading the graphic novel before watching the movie.
The translation from page to film is pretty accurate but there's too much context and subtext missing. I'd even suggest that having a good idea of how/why Watchmen is important in the history of comics and superheroes is necessary for full enjoyment. Don't get me wrong, I think the movie is well made and Zach Snyder did an amazing job, but I'm beginning to believe Alan Moore when he says that his work is unfilmmable.
I was worried for my fellow movie-goers who hadn't read the book. I was sure they would think the movie was terrible. Even as a superfan, I thought the pace dragged a little (but was pleased with how much wasn't cut) and the tone was too one-note. I think people are going to go into the movie thinking it's a superhero flick and halfway through, will realize that they're getting nothing of the sort. There's entertainment here, but nothing like Iron Man or The Dark Knight Returns. In comparison to the latter, I'd much prefer to re-watch Watchmen over and over though, because there's more you could unearth with multiple viewings, unlike Batman, who tries to give a nod to symbolism and meaning but is really just vapid in comparison.
In sum, the film version of Watchmen functions better as a visual companion to the book and doesn't necessarily work very well as a standalone piece. Don't watch it unless you've read it, that's what I'm saying.
Update: I take it all back. Watchmen is in (financial) trouble! Go watch it immediately. Then watch it again.