I had actually read the much-acclaimed graphic novel only last year. And while I found the story decent enough, I wasn't sure whether to like it or hate it. Undoubtedly it was an important point in the history of "superhero" comics, but I was never entirely sure about it's point.
So of course I saw the movie, and it had remained roughly 85-90% loyal to the source material, which made it a bit boring for me since I generally knew what was going to happen most of the time (except for you know, that thing about the bad guy's master plan... ). Not only does the story remain true to the comic-book style, but the backgrounds and choice of music seemed oddly appropriate for all the flashbacks going on to explain the Watchmen and Minutemen backstories. And the movie leaves me feeling the same way the comic did... ambivalent as it were, and unsure of what deep, thought-provoking message about human existence the Watchmen story was really trying to convey.
Still, if you're any sort of comic book fan, you should watch it even if you hadn't read the source material. The only real problem with the movie is that Dr. Manhattan seriously needs to wear more pants.