The switching back and forth in time was interesting but it seems like this has been overdone for me before.
My biggest complaint about Kate is that I didn't buy her ridiculous need for closure. My biggest complaint about Jake is that I didn't believe he could really be two entirely different people as he is portrayed in the book. However, the idea that his mom completely ruined his ability to maintain healthy attachments is an interesting one, I don't think it was explored enough in the book. The back story about Katie's parents' relationship was pretty interesting.
Like I said, overall, just OK.
One thing about these authors... overall I'm really curious about how two people write together. It's an interesting idea.