I have to say that I have ambivalent feelings towards Kingdom Hearts II's control scheme. On one hand, the controls are clean and responsive; I rarely encounter problems making Sora do what I want him to do, even in the midst of fast-paced boss battles. On the other hand, trying to execute any command outside of a limited set of commonly used commands has a tendency to get me killed.
I really like the controls for basic combo attacks and the movement abilities. With just the X, square, and circle buttons (alongside the control stick of course), it is possible to pull off amazing and flashy feats. The game benefits a lot from the context sensitive uses of the square button in particular. It can be used to cancel a ground combo in order to execute special attacks, to perform various special kinds of movement while running and jumping, and to recover from knock-back with a counter-attack (as opposed to a simple recover with the circle button). While it doesn't fall into any kind of clean category, having a specific button to perform so many miscellaneous functions works really well.
I also generally like the use of the triangle button as a dedicated Reaction command button. While I sometimes don't like the particular implementation of Reaction commands in Kingdom Hearts II, I love the concept. As seen in boss battles like Shadow Stalker/Dark Thorn and Luxord, it can add a lot to the game's design space when coming up with imaginative battles and spectacular actions. However, one of its best features is that Reaction commands are used to talk to people and open chests, which eliminates any overlap between those common functions and the game's basic movement/combat controls.
The problematic elements of Kingdom Hearts II's interface (which it inherited from its predecessor) are all found in the command menu. While the command menu gives gives the player access to numerous commands, such as magic spells, items, limit attacks, and special transformations, the interface is riddled with just as many problems:
1) Since the X button is techincally the button that executes the selected menu command, once the player moves the cursor off the Attack command to select something else, the player can no longer make basic attacks until he selects another command or manually resets it.
2) Selection in the command menu is controlled with the direction pad, which means that the player has to take his thumb off of the left analog stick (which controls player movement) in order to manipulate it.
3) The player has to turn his attention away from the game's action in order to manipulate the menu, without getting the chance to pause the action.
All together, this means that the player is helpless while selecting anything from the command menu. In a game where Sora can go from full health to dead in the blink off an eye against a tough enemy, even the second or two needed to select something in the command menu can mean the difference between victory and defeat. I know that I have certainly died nearly every time I had tried to heal myself from critical using a potion from the command menu. Fortunately, the command menu isn't completely terrible; it is laid out in such a way that extremely common commands such as limit are in very quick to access to locations on the menu. However, it is useless for any command that needs to be executed commonly or in a hurry.
To get around this problem, the player can set four magic spells and/or items to the L1 hot-key menu. By holding down L1 and pressing the face button a spell is assigned to, that spell can be used with the responsiveness that is necessary in Kingdom Hearts. Unfortunately, four is only half of what is needed in the game. It means that the player has to give up on using two or more of his spells at any one time. 90% of the game's interface problems could have been solved by creating a second hot-key list keyed to the L2 button, doubling the players number of usable shortcut commands. Allowing the player to make shortcut commands for Limits, Drives, and Summons would have helped a lot too (while there are ways to make those three kinds of command into Reaction commands, that interferes with the the ability of the player to recognize other Reaction commands).
All told, I would recommend to the developers of the Kingdom Hearts series to rethink the implementation of the command menu for the next game in the series. As is, I have taken serious damage or died way too many times while messing around with that menu for my liking. If the options embedded within the command menu could be accessed with the game's usual speed and precision, Kingdom Hearts II's controls would be perfect.