From what I've heard, it was very, very, different from the book, most notably in terms of the ending.
As I recall, the ending of the script was that Dr. Manhattan went back in time and prevented himself from getting superpowers. It also prevented Ozymandias from his evil scheme and opened up a hole in the space time continuem. As a result, Dan, Laurie and Rorshach (who survived) were transported to "our" world, where they discovered that they were characters in a popular comic book.
