Just one word to describe it: Great! Though it has faults it is the best of the best on the GBA.
The game has a card based system in which you choose cards that have a certain amount of card points that you have only a limited number of card points you can have so it limits the amount of powerful cards you can bring to battle. You can reload your deck in battle, but it takes longer to reload the more time you reload. By touching the heartless in the worlds it brings you to another screen to battle, where you press A to use the card you want and using L and R to sift through the deck.
To get through the world you need specific cards to get through the doors, to get in another room, which sucks.
The cuts cenes have PS2 graphics but there are only a few of those cut scenes. The graphics are great still though. The bosses can be hard at times but you just need to strategize and and get better cards. Also, after you beat the game as Sora you can play Riku's Story, which is the best part though you cannot choose your cards yourself.
The game is great, but a few kinks in the road is fine as long as it gives a great overall experience.