I am always leery when they take what I know to be a great book and turn it into a movie. You know the movie is never as good as the book. You can only hope that they come close.
The Harry Potter books were incredible. Not just good, but awesome, mind-blowing incredible. Not so much the movies. Its not any one's fault. It is impossible to pack that much information in a movie. You cannot develop the relationships in a movie that you can in a book. There isn't enough time. Unless you make it a mini-series with an intermission.
We just arrived home from seeing 'Harry Potter and the Half Blood Prince'. It was good. I know that all of those people (and their kids) who have not read the books will really enjoy it. I was actually surprised by how much of the book actually made it to the screen. But not all. How can you possible hope to impart all the complexity that is Severus Snape onto the big screen? He is worthy of pages of text delving into the mystery of what motivates him.
When I read the books, I laughed out loud, I gasped in shock and twisted in excitement. I cried. When I looked at my son during the movie, he seemed to enjoy it but it did not inspire the emotions that the books would have done.
I could go on and on but I won't. Go see the movie if you want. It's pretty good and has great special effects. But if you want spine tingling action with great dialogue and interesting, in depth characters....read the books.