Christopher Paolini began writing his first book at the very early age of fifteen, or so the story goes. While it is quite astonishing for one so young to write (and finish) a 500 pages book, it becomes less impressive when you actually read it. Certain chapters are simply dull whereas characters and the overall story are strongly inspired by heavyweights like LotR and StarWars. As an experienced reader of fantasy literature you will find nothing new in it.
However, the movie itself is "Fox 2000's version of my [Paolinis] story. I had no control over their decisions."(Paolini on the film), which is probably a good thing. They skipped the long travelling passages (and still have some amazing landscape photography) cut the boring dialogues and show us some nice special effects and costumes.
The flaws that remain with the movie are due to the fact that you can not turn Paolini in Martin/Eriksson/(insert your favourite writer here) and Eragon/Eldar not into Lord of the Rings/etc.
Die hard fans of the book(s) might be pretty disappointed after watching, but overall Eragon is neither the worst fantasy movie ever made nor the best.