In some cases it really annoys me and in others not. I watched the first 4 Harry Potter films before reading the books, and thought they were fantastic. When I then read the books and saw the differences, it didn't really bother me. When I had read the following books before the films, however, it did bug me a little. However, with books as long as Harry otter, I completely understand why it had to be done, so while I might miss a few things, I think they did an excellent job at adapting it.
There are other cases like that too, like the Wizard of OZ movie, which is very different from the book, but in my opinion, actually better.
When it comes to Disney's adaptations of fairy tales, I usually approve too.
I know thee are lots of films that I've hated for changing aspects of the books, but I can't think of them right now.