I'm almost always disappointed by the film if I've read the book. So when my favourite books get adapted, I'm excited and frustrated at the same time.
There were several films which I liked (for example, Harry Potter and that film about Narnia where they travel by sea). I also like adaptations of the books which I didn't like that much when I was reading them, in this case the film might be better. But usually I have my own pictures in my mind when I read something and they don't match with the film.
One of the larger disappointments I remember was Lord of the RIngs. I've read the book when I was 14 or so and I was a huge fan. I even made up my own alphabet based loosely on runes. I was reading and rereading the book, I've learned almost all the poems from there and even one of the prose parts. And then I came to my friend's house to watch the first film with her. I remember that it was rather good by itself, it was very colorful and I'd like it for sure if I didn't read the book. I wouldn't even remember what was so different from the book, even though there were some changes in the plot; the main reason was that I pictured it differently in my mind. So I wasn't able to watch it. And I didn't, I only saw the first hour or so. I think I might even like it now that I don't remember the books that good. Maybe.
And this happened almost with all the films based on books that I've read. Even the later Harry Potter movies which I certainly like, I try not to associate with the books because they have a very different atmosphere. One of the strangest adaptations I've seen was The Hobbit; as far as I remember, the plot of the book and even most of the lines were the same but the atmosphere was so very different.
Another film I can remember is Golden Compass, based on the book by Philip Pullman. I loved the books when I was in school so I decided to watch the film but I wasn't even able to finish it, it was so different from my imagination.
I think the problem is that films have a different way of telling stories than books. Books can contain more details, most of them can't be put into a film because there's no time for that. Of course you don't want to watch a part of the story if you've already read it all. There are also films which use visual effects to attract people, even if they're unnecessary at that particular moment. And I like books more than films in general, I only watch a couple of films each year so some films are good but they're films and thus they're not as good as any book might be.