I think it depends to a large extent on how attached I am to the book. I tend to be much happier about watching films of books I liked or thought were okay than I am about watching films of books I love.

Hence I was quite happy to watch film versions of Jurassic Park and The Lord of the Rings, but will be avoiding the films of The Time Traveller's Wife, Jonathan Strange & Mr Norrell, and His Dark Materials like the plague.