"Watchmen" the graphic novel is an acquired taste. It is too static, too wordy, sacrificing some of the essential advantages of the medium. To make things worse, the drawing style of Gibbons is boring.
What gives "Watchmen" a place in history is Moore's relentless script. One of the images that keep appearing in the comic is clockwork. It's the perfect image of the story: a perfect clockwork, a machine where every part works together to reach the inexorable conclusion. The term "graphic novel" was redeemed by Moore: he actually tried to write a novel.
Of course he didn't succeed. Novels are different animals. His achievements are firmly in the domain of comic books: an incredible anti-hero (Rorschach), a magnificent villain (though Grant Morrison wouldn't agree with me), and a couple of wonderful plot twists.
Finally, while "Watchmen" are a tour de force, they are not as enjoyable as some other works by Moore. "Miracleman" is more passionate and groundbreaking, "Swamp Thing" is more poetic, and "V for Vendetta" is more perverse.
Still, the most impressive thing about "Watchmen" is the sheer amount of love, energy and thought invested by a very smart man into a medium that was, and still is, considered shallow.