First, a disclaimer: I have not yet finished the game.
The game is part relationship simulator, part RPG, part "trading card" type game. The RPG side does not require you to get overly in-depth with your characters to get through the game. However, if you like getting deep, the "Personas" feature a "fusion" option that allows you some level of customization for the magical powers you carry with you.
The relationship simulation is actually an important part of the game, in that the deeper your relationships are, the more levels you gain when you fuse Personas. The sim part of the game is also a good counterpoint to the grinding of the RPG battle part of the game.
While I like the combination of these types of games, there are some minor issues, such as ally AI, "unfairness" on the part of the enemies, and a lack of save points in some key areas.
I can sum it up by saying this: I've been playing for 50+ hours, and can't wait to play the remaining 20 or so, not to mention replay value. And thus far, the story isn't as dark as it might seem.