What I liked: Olive's experience with OI (informed by the author's own) is believable and informative and Olive herself is bursting with personality, the slow burn of magical realism grows on you in a really interesting way, the realistic representation of small town life and church culture without being dogmatic or evangelical, Olive's journey to better understand her step-brother and what is going on in his life. 
What I didn't like: The story was somewhat predictable at at times it, and Olive herself, came across as painfully saccharine. It almost felt like a story and character written for adults' ideas of tweens rather than tweens themselves. I wish Olive had been infused with just a little more actual 6th grader angst/personality. Because of this, I think it may have limited appeal to middle grade readers and would fit best for the lower end of that range: 8, 9, maybe 10.