I bought my 8 year old daughter the hardcover box set for Christmas. When she opened it her initial reaction was definitely "oh...thanks" (she was clearly not excited about it but wanted to be nice). Within a week it was borderline impossible to get her to put them down and go to sleep at night.

Exact same story here. I got my father in law the box set as a gift, and when my daughter was about seven she started reading them when we were visiting them. So I bought her a set of her own. She still reads them all the time at 11.