I've never read the graphic novel, but a lot of people said that the Watchmen movie was very close to the original source.
They change one pretty major event that ends the film. It's significant and huge, but it might have seemed too far out of left field for audiences to buy. Now, I think they could handle it, since we've seen so much weird superhero shit, but at the time, it may have been WTF.
When people say that Watchmen is close to the original source, it's really more about the layout and order of the scenes, the framing. They literally use the graphic novel as a storyboard. And with Zack Snyder's speed-ramp style, the action often slows down and "locks" on the iconic comic frames. It was cool, for sure.



I will spoil the comic vs. movie thing if you want me to.