Though this is my personal second favorite game on the GBA, I would only truly recommend it to Kingdom Hearts fans.
Picking up immediately after the end of the first game, our protagonist, Square-created Keyblade master Sora, is wandering through a field with his friends Donald and Goofy of the Disney universe. A cloaked figure leads him to a castle in a rift between the realm of light and dark, wherein he finds that nothing is as it seems. The castle is controlled by the powers of cards, memories, and a mysterious organization intent on taking control of Sora's power. As Sora and his companions proceed to the top of the castle in search of lost friends, they use cards to unlock rooms formed from their memories and eventually find that following the chain of their memories can lead them astray.
Unfortunately, CoM is largely filler between Kingdom Hearts and Kingdom Hearts II. As more than half of the game involves visiting worlds from Sora's memory, the material in the game is primarily recycled, although there is a surprisingly engaging story which unfolds between the card-worlds.
Everything inside Castle Oblivion is controlled by cards, including battle. The card system has a rather steep learning curve and is ultimately not as satisfying as one might hope.
Technically, the game plays quite well. The frame rate may lag later in the game and the sound quality is a weak point, but this isn't much of a problem. The game contains some overly catchy tunes, many of which you'll recognize from the first game, and looks quite good overall.
One fantastic feature of the game is the ability to play an entirely different side of the story once you finish it the first time. In "Reverse/Rebirth," you will move more quickly through some of the recycled content and focus on the heart - pun intended - of the story. The new playable character also keeps things interesting.
Though Chain of Memories is highly recommended for fans of the Kingdom Hearts franchise, casual gamers might want to avoid this game because of its steep learning curve and slightly disappointing card system.