I have seen some pretty decent book-to-movie adaptations, and then I have seen some real 'stink-o' jobs.
First place would have to be the bizarre treatment of "Mists of Avalon". Second would be "Clan of the Cave Bear". Both of these would be hard to guess what book they were from, if you didn't know ahead of time.
I liked the work they did on "DaVinci Code" . (I missed "Angels and Demons" so don't know about that one yet.)