Chain of Memories is just a chain of crap. Play the PS2 version if you want to know the story and play the game better.
It's hard to play, the Cards are just plain horrible. Half the time you will be more button mashing and less stocking. Sleights are gonna be less important and CP will. It's pretty good if you like dungeon crawlers (The only one in the series), but is horrible. Do yourself a favor and play the PS2 version before you hate this.
It's still fun to play, it is still a dungeon crawler, it's story works, and it has the best sprites i have seen for a GBA game. The cutscene could use little voices though.
Sora's Story is great, but once your done, play Riku's Story. His story is better and more fun. Strange, Sora's Story is harder D:
It is both a complete mess and a huge blast. Get it for your GBA or just emulate it. Either way, don't spend the money on this game unless you REALLY want to. Get Re: Chain of Memories for the PS2 instead.