Well, my only experiences with this was when I watched Bridge to Terebithia and then had to read the book for school, also when we read Helen Keller and then watched the movie. When I started reading Bridge to Terabithia I knew everything that was going to happen exactly, but it was still a good book. I haven't read Lord Of The Rings or Harry Potter and I thought the movies were really good. But people who read the book probably thought it was better.
The reason for this is that when you read you can let your imagination run free. The characters can look like whatever you want, and the images you develop from words are your own. A movie doesn't allow as much imagination, but you have picture, sound effects, it is a lot faster and it can be more realistic (you see the people, instead of reading about them). When we read Helen Keller I got a very distinct image of what I saw the characters as. When I watched the movie no one was how I thought of them and the movie had a much different feel than the book.
I enjoy reading books but my problem is if I am reading a book and I travel somewhere or for some reason take a break from my book I never usually start reading it again. When I watch a movie I always finish it, and I can finish it in one sitting. Another benefit of watching a movie over a book is that it can almost feel real. Especially now with 3-D when you watch a movie the sound, picture, music, and 3-dimensions really bring it to life. Surround sound can make the movie sound like it is really happening all around you and your in the centre. Music can very easily set a mood. It can help with tension, thrill, and emotions. The picture really makes you think of the characters as real people and you can easily connect with them. Also picture can vividly show gruesome, or detailed scenes. 3-D really put it over the top. The characters look like they are standing right in front of you. It feels like you can touch them, and like you can interact with them. It also makes you think that you are in the movie.
When people watch a movie after the book they are generally disappointed because often what they imagined isn't on screen. If someone watches the same movie without reading the book they may find it amazing, because they have no mental comparison. Perhaps it is not the adaptation itself, but the expectation one may have.