If you lack imagination and the ability to suspend disbelief then you probably will not enjoy this series. I've seen reviews stating that the ending was a total shock and came out of nowhere. It absolutely didn't! There was a slow build-up and the end was horrifying but there were clues throughout the series.
I thoroughly enjoyed it, David's glaswegian accent takes some getting used to (very guttural and harsh, unnerving especially for a psychiatrist) but apart from that small issue it's an enjoyable series. I do hope they make a second series.
Now I'm off to buy the book.