Not as good as it could have been. I'd mark her report card 'Must try harder'. It reads like a slightly expanded magazine article; it could have been a lot more. There were three 'patients' and four exorcists covered, only a couple of which featured throughout. Not enough really. There were one or two sceptics quoted, a couple of scientific types, but all-in-all it was a bit of a disappointment. Given the subject matter, there's a lot more fantastic stories to pursue and cover. This reads like a journalist having nothing else to do on a Sunday afternoon but interview a couple of priests, a policeman, a psychologist, and three women.
The accounts of the exorcisms were quite dry, and almost boring in recital. The conversations with the priests were incomplete, where comments such as "You can't see what I've seen and not believe the Devil is an actual living person" were just left like that. No further elaboration or explanation. It leaves you feeling that there are much more interesting and elaborate tales out there, but that she just hasn't bothered to pursue them.
She made no real decision at the end, and given the paucity of evidence she appears to have garnered, you can't make any decision at all on any of the stories based purely on the information provided. So you're left with your own conclusions, which would be exactly the same ones that you started the book with.
It is an interesting subject, so not a complete waste of time, but it leaves you wanting far more in depth analysis, stories, and evidence for and against.