Never judge a movie by a book. They just aren't the same thing. Though I am of the opinion that movies shouldn't be based on two things, true events and books. Neither makes for a good movie because the movie format just doesn't lend itself well to such things. True events just don't translate well to the screen and in order to get them to do so, facts are lost/change. A documentery is always better. Books are simply too long and just too different of a medium to translate to a movie at all. At best you can do a mini-series to do justice to the full story.
Movies simply need to start out as movies. So yeah, to make Eragon or any book into a movie, the writer starts out knowing full well the story will be bastardized. In the end the movie script is simply something that was inspired by the book. Maybe I am an old fuck, but I stopped the whole, this movie better be as good as the book long time ago. Let the movie be a movie and judge it independant from the book. Or just don't read any books so you can enjoy any movie.