I think for me it depends on what the adaptation does violence to whether I can live with it or not. The godawful movie travesty that was (or rather wasn't) _The Dark Is Rising_ thoroughly irritated me, but I loved the second Narnia movie, primarily because I felt it modified/took issue with some of the things that bother me about the book and about Lewis' writing in general. I haven't seen the third Narnia movie yet, so I can't comment there.