My biggest fear concerning this movie is that it will come to be associated with the name "Watchmen" more than the graphic novel, which is indeed deeper/better (as with MST3K, the right kind of people will "get it").
I highly suggest reading the book before seeing the film. As for Dr. Manhattan's big blue junk...I'm hearing that it's the one thing people remember most about the movie. A shame, really, as there's so much more to the character than the juvenile response of "OMFG, a penis!!!" that I've heard IRL several times since the movie was released.