He's publicly denounced the movie, even though he's never seen it. To be fair, he gave "V for Vendetta" a chance, and while it wasn't necessarily a bad movie, it butchered the source material. He gave "League of Extraordinary Gentlemen" a chance, and holy crap was that awful.
I honestly feel like that if he gave "Watchmen" a chance, while I'm certain he would still probably hate the film, he might at least appreciate the effort that Zack Snyder made to stick to and honor the original source material. Personally I loved the movie. Some people have issues with it, mostly how the ending was changed, but I understand why it was done and I don't have an issue with it. Trying to explain that Veidt bioengineered a gigantic alien squid to an audience who had mostly not read the comic would have been nearly impossible to do within three hours. People just wouldn't get it.
In short, I really hope Moore at least gives it a chance some day. If anything, he might enjoy Jackie Earle Haley's portrayal of Rorschach.