It's an awesome game but at times it can be a bit frustrating.
Gameplay: I truly think the card-battling system couldn't have been better but the battling system definetly didn't have to be card-based. Actually I might want to scrath that since the game revolves around cards so making it card based in turn made the game better. It added a challenge to the bosses. It's also like I said a tie-in with the story.
Graphics: The Graphics in CoM are awesome. I rarely see something this good on the GBA, although I don't play my GBA often. All I can really say.
Sound: The sound and music in this game is awesome. It plays the music of the Disney movies that are featured in this game. As well as its own music created just for this game and its prequel.
Value: The replay value of this game is good too since you get to play as Riku in Reverse Rebirth Mode once Sora's Story is completed.
Overall if you are a KH fan you owe it yourself to play this game. And if you just want a fun action/rpg to play for the gba this is as good a choice as any.