If you like the book then you will love the video game.

The first and second movies were really good..I recommend them highly..and normally hate seeing movies of books I've read and liked, because they almost never live up to the books..but these too are excellent...

There were many things in the book which didn't make it into the movie, as usual.
However, I'd say that the movie was a pretty fair adaption of the book.
Some things were definitely changed based on artistic license, and time constraints.
If you like Puzo's writing, I'd recommend reading "Omerta: A Novel". It was good, too.

I first saw "The Godfather" at a drive-in movie, when I was sixteen. But early on in the film, when Tom tells Sonny his dad wants to see him (and Sonny is 'hooking up' with a woman, while standing up) my girlfriend and I climbed into the back seat of my jeep, and got into some heavy 'hooking up'. So much so, we never saw the rest of the movie. So, I had to wait for it to begin appearing on television, before I actually got to see it in its entirety!