I can say that the book is richer than the movie. Where LOTR and even HP movies have left out about a 1/4 of each book, Eragon movie left out about 3/4 to give an idea. The book was shaved so thin it's see through. They didn't even have the werecat!!
It was a major disappoint to me. It's near impossible for me not to like a movie. At least it was enjoyable spending some time with my girls. I so wanted it to be good.
My suggestion-borrow the book from the library and rent the movie.