the harry potter movies are a prime example of how NOT to adapt a book to film...all the movies so far have been pretty bad. of course they cant possibly include everything, but i just think the way they are made takes so much away from the book. virtually nothing in the movies is how i imagined it when i read the novels, which i guess could only be expected but it kinda ruined it for me.
having said that, some books do make good movies. i think classics like pride and prejudice, emma etc for example make very good films, and like someone else said, funny novels like bridget jones diary sort of lend themselves to the physical humour that can be used in film. but for the most part, i steer clear of film adaptations unless i either really love the book, or i havent read it at all.