It is the best game on the DS, except MAYBE the Layton games. It's also the best game Squeenix has made in a decade.
EDIT: The plot is excellent. I originally thought that, due to the age and colour/art styles, it was going to be a childish kiddy-game. While some of the common RPG themes targeted at young people, like 'making friends' and 'being yourself' and 'not shutting people out' and that sort of thing are common throughout, the storyline is actually quite dark and creepy, with a lot of great twists and character development. The game is divided into chapters, and after you beat the game, you can go back and replay chapters with special side-objectives. These objectives reward you with extra, unique items and so on, but they also give you access to "Secret Reports", written by another character who's identity is unknown. It adds a series of new layers and twists to the plot.
The gameplay is quite strange at first, but you get used to it quite quickly. If you're a completionist like me, you will spend AGES grinding up your pins to make them evolve and to earn enough money to buy every item.