Books vs. Movies

You ever find yourself reading a book that you've already seen the movie of and thinking to yourself 'This would have been so much better if they added this instead of cutting it out. Now I understand the character/scene better.' Well that is normally what happens to me. 
One night, hanging out with friends, we decided to watch Stephen King's IT. Before I got to finish the movie, it was time for me to go home (i.e. parents were at the door as I was too young to drive) and never finished the movie. I read IT and with the first page I was hocked. It was so descriptive and gruesome that I couldn't put the book down and just kept reading (remember this isn't a short story, it is over 1,000 pages). While I was reading IT, I remembered back to the movie night with friends and Tim Curry giving us his creep smile as the clown who liked to float, everything floats. After reading the book (which managed to spoke me) I watched the whole movie and thought it was crap. The acting was good and the cast just the same, but the story line was nothing compared to the book. The book spooked me for a week or so and I was expecting the movie to have me just as 'scared' for at least a week. By the end of the movie, I was disappointed. I love Stephen King books because of his 'what if' attitude regarding the supernatural. 
Currently I am reading Stephen King's Secret Window, Secret Garden. I have seen the movie and thought it was brilliant with the writer going crazy, coming up with this mysterious person and doing terrible things while he thought he was asleep (including killing his own cat, burning down his ex-wife's house and eventually murdering his wife) and in the end we find out that it was the writer all along (kind of like Fight Club, but instead of fighting, they are writing). I have just come to the part where we (the audience as well as the main character (Mort)) find the cat with a screwdriver nailing it to the garbage lid. My only hope for this book is that it is as brilliantly written as it was portrayed in the movie. 
As a final thought, I do normally try to read the books before seeing the movies as I find my imagination is a lot more twisted and dark than what is normally portrayed on film.