Ask 10 serious movie people who the best living director is, and you'll probably get 10 different answers. Ask 10 serious comics fans who the best living comics writer is, and you might get just one: Alan Moore, the toweringly tall, vastly bearded Englishman who wrote Watchmen. (You'll also get 10 lengthy explanations of why that's the case--there is no bore like the Watchmen bore. I should know. I am one.)
So it is with a queasy, not un-Judas-like mixture of devotion and betrayal that the hard-core Watchmen fan approaches the movie version--Jesus in this scenario being Moore. He has not been well served by film adaptations, and he is resolutely hostile to them. Not only has he refused to see the Watchmen movie; he has refused to be paid for it. Moore, who likes his words unminced, told the Los Angeles Times, "I will be spitting venom all over it."
Personal respect for its creator isn't the only reason not to see Watchmen. There are aesthetic grounds aplenty. The book doesn't lend itself particularly well to film. It's a long, many-threaded serial narrative that's not meant to be forcibly administered in one dose. Its content is also not easily extricable from its comic-book form. The fifth chapter, "Fearful Symmetry," unfolds symmetrically, the panels at the beginning echoing the panels at the end, with a grand mirror-image spread at its heart. Palindromes, reflections, symmetries--Watchmen teems with them. Look at Rorschach's face. They give visual life to the tensions that animate the story, between the chaotic flow of time and the perfect frozen moment (each panel is one), and ultimately, between good and evil.
Bottom line: this is about knowing what you're getting into. The mistake for newcomers would be to confuse Watchmen the film with Watchmen the graphic novel--to think of the film as a substitute for the book. The two are neither identical nor symmetrical. The film is an homage to the original or perhaps an advertisement for it, but nothing more. It is not it.
Should the fans see Watchmen? The comic has something to say on this question. Are you an idealist like Rorschach, who insists on absolutes, black and white? Or a pragmatist like Ozymandias, who deals in shades of gray? Ozymandias would go. Rorschach wouldn't. The point of the comic is that neither position is perfect or even tenable. But a choice must be made. To quote the master's final words: "I leave it entirely in your hands."