My wife and my office manager were really into that series, until the final book. They both absolutely hated it. Supposedly over 25% of advanced sales have been returned due to the dislike of the book. From what my wife said, the story is not all that bad but she broke every rule she had set up in the first three, rules she stated in interviews before the release of the fourth that she did not break. My wife also said that the writing style seemed to be from a different author and had very little in common with the first three.
So many people are mighty upset with how she ended that series.