I had never heard of Watchmen until I saw the trailer at TDK, and immediately went out to buy the book. It just looked so awesome, I jumped in with it as my first "graphic novel". I'm glad I read it though, as the movie was supremely mediocre. Interesting that the worst thing people have to say about it is usually that it stuck too closely to the story (excluding the ending obviously), which usually isn't a bad thing.
Rorschach, The Comedian, and Silk Spectre were so incredible looking and seeing them come to life was one of the most fun times I've had at the movies in a while, so much so, it almost made Dr. Manhattan and Veidt tolerable. The most outrageous and egregious mistake though was the soundtrack. If they just kept the movie to a score it would have kept me interested, but everytime I heard Hendrix or Leonard Cohen or any of the many modern classic rock songs, it completely took me out of the story (the one exception being "Times They Are A' Changin'" for the titles. That at least seemed appropriate.)
Oh. And what a ridiculous Nixon.