Well, it depends on what they do with it.

There are times it deviates, and I go nuts.  Demi Moore's version of The Scarlet Letter is a perfect example of film that I pretty much just watched slackjawed because I couldn't believe what I was seeing.  Whiteout was another.

There are times when they are faithful to the book and it doesn't help because the book was so stupid (Battlefield Earth).

But there are times when the changes are radical but can actually help.  I hated the book Stuart Little, I thought the central character was a selfish dipstick.  The movie changed him and made things far more tolerable than the book did.  Shrek was completely different from the book it was based on.  Aquamarine made the character likable -- still not a great movie, but you were actually pulling for the character.

So change isn't the bad thing, it's what is done with the change that is the question.