Most of all, I really loved the main character - Carolina "Nine" Livingstone - she was not at all clichéd in describing her feelings, instead her unique take on things made the book interesting for me, eg. her references to space and weather in eloquently put ways.
In terms of the storyline, I think it was not the run of the mill love-at-first-sight realtionship, but built up the emotion between the teacher and student over the book. One example of the couple's unique charm is things like their love of Emily Dickinson poetry and the fact Nine is taller than Mr Mann. I think the story becomes heavily weighted on only telling the story of Nine and Mr Mann but this is well told, so it makes up for it.
Overall, I recommend this to teens or hopeless romantics, since the close scenes between the teacher and student are told with a lot of gut feeling, or people who like emotionally charged stories, as this one certainly has plenty of grit in places.