Ehhh, I liked it and found it hard to put down but its a bit gruesome for me. Even when I was expecting it to me. I also had a really hard time liking any of the characters in this one. Probably won't be finishing the series as I believe there are many.
This is the perfect book for those who like dark thrillers (not necessarily mysteries) but those about serial killers and kidnappings. It can be hard to stomach at times. I have to say I liked Slaughter's most recent book (which is the only other one I have read) more than Triptych. I think it must be the characters.
It has a lot of good reviews on Goodreads, so I definitely recommend trying it if this is your genre (I'm trying to expand).