In the case of this movie, I'm glad I wasn't familiar with the graphic novel beforehand. It seems like a lot of people who've read the novel are having a hard time reconciling the differences. The one thing that most people agree on is that the opening credits were a masterpiece. I'm thinking that's because they were new to everybody.
I agree. I hadn't read the graphic novel in 5+ years, and purposely didn't pick it up before watching the movie, because I didn't want to be analyzing the movie, comparing it to the GN, etc. And, except for a few things, I loved the movie. All of my friends who did the same enjoyed it too.
After re-reading the GN after watching the movie, I was reminded of a lot of things that I wished were in the movie. But it would've been impossible to do under 4 or 5 hours.