There are only a few films adapted from books where I have read the book and seen the film. Technically the count is 12 but it feels more like 3 as 8 are Harry Potter, 3 are Lord of the Rings and the other one of Where Eagles Dare. When The Hobbit is release that will be another 2 and if the film version of Foundation goes ahead that will be another one. I also have a copy of Do Androids Dream of Electric Sheep but I've not read that yet. So for now it's just twelve. The thing is, contrary to popular theory for Lord of the Rings and Where Eagles Dare I kinda prefer the films over the books. Reading Where Eagle Dare I could never quite grasp who each character was and constantly got them mixed up. It was much easier seeing distinct actors in each role. Although I was a lot younger when I read that. The Lord of the Rings books are great but the films just seem to be even greater. The story seems much straighter flows better in the films (extended). The books tend to get a little slow and off track at times.
Now perhaps it's due to how soon I watched the films after finishing the books but I did not feel the same for Harry Potter. For those I thought the books to be infinitely greater with the possible exception of Deathly Hallows Part 1 oddly enough. There were so many little details missing from the films that I found myself wondering how someone who had not read the books would understand. However my main problem was with characters. The core of the cast was okay, Harry, Ron, Hermione, Neville, Luna, Draco, Dumbledore, Snape and Sirius were all fine because I knew which actors where playing them before reading the books. However many of the other characters didn't meet the expectations I had in my mind, mostly the Weasleys in particular Arthur, Molly, Fred & George. Also how can you leave Bill out completely until the Deathly Hallows when he's the best Weasley.
Now for why Deathly Hallows Part 1 was better than the book for me. I think it's simply that in my mind all the various forest locations looked fairly similar and it didn't feel like they were getting anywhere particularly quickly. However in the films they've sped things up considerably and found lots of the lovely looking locations to shoot in. In general I did feel that Deathly Hallows parts 1 and 2 were the best of the films. I wonder if that's because the extra length allowed them to fit more of the book in or if the VFX were more matured or if the acting was better as everyone has gotten older? Probably a little of each.
That's my Harry Potter marathon completed now though and it has left me as a fan.