I bought 20th Century Boys prior to the Expo and also loved it. I've read the entire manga and couldn't help comparing the two though. The movie is an amazingly good adaption but I didn't like how some key scenes had been changed and also the character development is seriously lacking when compared to the manga. Of course the movie can't include everything otherwise it would never end, but there were some scenes that just needed to be in the movie, only they weren't. I was quite disappointed to see some major scenes missing because I personally think the film would have been better if they were included. I urge anyone who has the time to read the manga before viewing the film. It is one of the best series you will ever read. I feel that the film will reach it's conclusion too quickly, despite it being a trilogy. As mentioned, a lot of major develolment is missing and also the suspense and tension is nothing compared to that in the manga. It probably sounds like I'm some serious hater of the movie, but don't worry I love the movie to death, only the manga is one million times better.
I'm glad I read the manga first because finding out the ultimate truth throughout three films is no where near as satisfying as completing all twenty four volumes of the manga. The film needs six parts at least so it can reach the same level of suspense and anticipation the book did!