Overall, the story is cute. A boy with a weak heart goes to the country to recuperate, in a house where a family of tiny people-creatures lives. The Borrower girl, Arrietty, wants to explore the bigger world, but puts her family in jeopardy by befriending humans.
There's a dark sort of undertone to the whole story: the boy is going to have an operation and is obsessed with dying; Arrietty's family lives in isolation, not knowing whether or not there are any other Borrowers like them left; the human housekeeper is a bizarrely controlling and manipulative "caretaker". And then there's the one little "wild" Indian-looking Borrower who runs around with face paint and a bow and arrow, which was not cool; this is 2012, and that sort of imagery is a relic from a more unenlightened era.
As a coming-of-age tale, though, I thought it was aptly unresolved: both Arrietty and the boy learn that change is one of the only constants in life, and that their survival depends on that realization.