I don't like books that telegraph their endings.
O'Farrell tries her darndest to keep her "surprise" ending hidden from her readers, while giving tantalizing hints through a series of flashbacks (from the mind of an Alzheimer's patient, which does make it a bit more interesting). But, really, anyone who's read consistently over a period of a dozen years or so is going to be able to figure this one out.
Iris is a seemingly content woman suddenly burdened by a great-aunt she knew nothing about who has been in a mental hospital for the past 60-some years. O'Farrell does an extraordinarily nice job of characterizing the aunt, and describing the symptoms that committed her in that day and age. But she does a very poor job of "creating" Iris for us--Iris' back story has seemingly no connection to the back story of her great-aunt. It's as if O'Farrell is trying to create something additionally shocking to obfuscate the surprise. Feels contrived.