Mediocre all the way
I'll start off by mentioning the few good things about this game. The graphics were probably the best into 2002, and they look good even today. The music is also good, but lasts about 10 seconds, and it'll get old really fast.
Now for the bad. The biggest is the platforming. Sora can easily fall right off platforms, especially when you attack an enemy atop one. Also, allies can stand on the platform you want to jump to, making it impossible for you to get up onto it.
The Disney characters completely surpress the Final Fantasy characters. Square might as well just not been included in this game. The Disney characters are the only ones who can travel in your party, and the only ones who can even fight. I thought this was supposed to be a joint effort.
The gummi ship is a joke. All you have to do is hold the fire button, and you can get from world to world just fine. Very boring.
The combat, like the gummi ship, is all just button mashing until either you die, or they do. You can't control your allies, and they just sit there and take damage. The combat is also very hard because of this, since all you can do is run into the fray and start pressing, and you'll find yourself repeating levels too many times.
I don't know why anyone else like this game, but I certianly don't. Kingdom Hearts does not like up the the hype, and is mediocre all the way.