Read the graphic novel first. It's a classic of the first order. It basically created modern comics storytelling. There are twists in the comic that are very clever and well foreshadowed that the movie can't possibly match simply due to length.

I'm definitely interested in seeing the movie, and I don't think that it will be bad, as many fans of the comic do, but there's simply no way that it will be able to live up to the comic itself.