You know, I was told that the manga was much more in depth and that the movie cut a lot. I was also told that that made it "bad".
Frankly, having read the manga now, and after seeing the movie a few times, i think the movie actually does a fantastic job at gathering the main plot points into 2 hours and change - It's almost a cliff's notes version of the manga. Yes, you miss a lot, but much of what is cut out is secondary plots, extended developments, tertiary characters, flashbacks, etc. I definitely recommend a read, however, because it fleshes out a lot more things, and you when you re watch the movie you end up seeing a lot of those cut characters in minor scenes.
You can read the manga online here, if you can't find it in book stores.