I had heard that the plot was hard to follow and that you need to have read the graphic novel, but I hadn't read the graphic novel and I had no trouble following it. Also, don'tbe put off by the length: I found it engrossing all the way through.
Just like Hancock, this film got poor reviews. (On Rotten Tomatoes, Hancock got 39%; Watchmen got 64%.) It's hard to understand how two such good movies could get so panned by the press. It may be that critics don't "get" superhero movies: they think they should be shlock, and can't handle complexity. Unless, of course, one of the stars ODs before release.
(If I had guts I'd have written about the social implications of Dr. Manhattan's genitals.)