The plot was altered heavily from the novel, but even the novel had a subpar ending. It was a smart decision on Jodie Foster's part to stay away from this film.

I liked the film, but then again, I haven't read the book, so I have nothing to compare it to. It was a bit odd having Julianne Moore as Clarice instead of Jodie Foster, but I thought it was pretty good.

^
Jodie Foster opted out of the film because originally they were going to do the book's ending.
Without spoiling much, the book's ending has both Clarice and Hannibal behaving exteremly OOC.
In an effort to try and bring Jodie back they changed the ending. Jodie still would not return because the rest of the content still made her uncomfortable, as Hannibal has basically become the only person she can trust anymore and given Jodie's past, it really hit a nerve.
I wasn't all that thrilled with the movie even before I read the book, mainly due to the ending (although it was better, it still left a lot to be desired) and that laughable brain eating scene (which was also stupid in the book).