Absolutely chilling.  The fact that unstable, psychotic killers can blend into society so well is what made this story so scary for me.  The writing was fast paced, the action suspenseful, and the plot very plausible.  Dan's character and escalating instability was very well written.  I do think Melissa's character could have been fleshed out a bit more as I really didn't get a very good understanding of her.  Just when I thought the story was finished and I felt safe again, thinking Dan and Melissa could settle down into their relationship instead of running for their lives, here comes a surprising gut-punching twist that left my heart racing.