It depends. As long and packed with semi-pointless backstory as the movie was there is quite a bit more in the book (and with enough extra that it is no longer semi-pointless). For example, you'll know enough about the people involved to actually care about
The ending is different. Better in the movie, in my opinion.
That stupid lion thing won't seem completely out of place (they should have cut it since it was completely without explanation).
A lot more detail on Rorschach, which is good.
All in all, I'd say that if you were intrigued then read the GN. It certainly wouldn't be a burn. But at the same time I was flipping through it last night and it is remarkable how much of it is just a shot for shot storyboard of what was in the movie. So I might suggest waiting a couple months for the movie to fade.
I'm not a big fan of the GN but the movie did make me appreciate it more in that the huge flaws in the movie shined a light on the subtle good things about the novel.