Completely for fans of the series and those who have already beaten the first. Fills gap between KH1 and KH2.

This game had odd releasing because it wasn't released in Japan and a sole game, but as a bonus for the incredible Kingdom Hearts 2 Final Mix +. The game is a great transition from the Gameboy Advance version to a Ps2 version. It boast different skills, voice acting and motion scenes (which all weren't in the original due primarily to the fact the gba couldn't possibly handle it). This should ONLY BE PLAYED IF YOU HAVE BEATEN THE FIRST KINGDOM HEARTS. It fills the gap between KH1 and KH2 when Sora and Riku first get involved in the Organization XIII schemes (which i won't spoil).
Once you beat Sora's story in the game, you can play as Riku for the first time on Ps2 (besides a fight in KH2), which is very refreshing especially if you are a fan of Riku.
All in all, if you are a fan, there is no reason not to purchase, and if you are not a fan, play part one and become one.