I think I've only avoided one film because I loved the book too much.
That was John Irving's 'A Prayer for Owen Meaney', which shows up on a lot of peoples favs ever. It was made into the film 'Simon Birch', and you could tell from the reviews that it didn't really 'get' the book, so I avoided.
I really love Alan Moore's 'Watchmen'. Even though the trailer looks excellent, and obviously adapted by someone who appreciates the book, I'm still not sure it can be compressed into 2 hours.
Any films you wouldn't watch because of your love for the book? Time Traveller's Wife, for example?