Some reviewers state that one must read the book to truly understand this HP film. I agree a little with this thought. I am an avid Harry Potter fan. I have read all of the books a few times. There were parts in this movie (as with any of the other HP movies) that I thought someone who never read the book would really not "get." The books are fantastic, so I would encourage anyone to read them.
The movie followed the storyline of the book quite well. The screenplay 'doubled up' some scenes from the book. I thought this was nicely done.
The CGI was pretty good. Nagini could have been a little more convincing, but oh-well.
I am excited to see part II. Most of the action does take place in the last half of the book, so it makes sense that the movie would follow suit.