I saw the movie when it came out, and I love it. It's one of our few traditions that we (or at least one of us) watches it ever November 5th, and we've done so since the first November 5th it was on DVD.
Anyway, I didn't actually know it was based on a graphic novel for about 2-3 years, then immediately snagged a copy when I found it in a bookstore. I've been trying to read it ever since, but kept failing to get very far. Part of that, possibly the biggest part, is that I wasn't really thrilled with the art. It's not really bad, it just left me feeling pretty "meh" about it. I also had a hell of a time reading and following the story because of the font style (I had to squint and re-read bits on almost every page because I couldn't make out what it said) and some of the dialogue that was written sort of phonetically to convey the accent or whatever. That kept pulling me out of the story and minimizing my interest.
The story itself, though, was really great. I think I would have liked it better with some changes (like text that was easier to read), and I doubt I'll be re-reading it anytime soon, but I didn't hate it and I'm glad I've finally read it. I was really surprised by how closely the movie followed the graphic novel. Usually there are a lot of changes, or at least that's what I've noticed from my limited experience reading comics and graphic novels, and seeing the movie based on them. This one, though, was pretty similar. The movie was so well done, I would go so far as to say to not bother reading the graphic novel if you don't have a very strong desire to, because there weren't many changes, and most of them were so insignificant it doesn't change anything major.