I thought HG was a very good adaptation of the book, especially considering how many different ways they could have really messed up the movie. That said, there are a number of nuances in the book that didn't really make it into the movie. I lost track of how many times I saw something and immediately thought "someone who hasn't read the book won't really understand the importance of what just happened..."
Not saying one can't follow or enjoy the movie without first reading the book, but I think someone who's read the book will be able to better follow the movie.
[Oh, and don't bother with the other two books in the series]