If you're a fan of Square or Disney you can't go wrong with this game.
YAYS
Lots and lots of Disney characters make it charming; simple but fun battle system; challenging enemies and boss fights; loads of secrets; pretty graphics and music; so many things to do
NAYS
It's more Disney than FF - almost all of the FF characters have little or nothing to do with the story; Atlantica is one terribly designed level; button mashing can get mundane once your character levels up; story is too simplistic
If you're a Disney fan, or a FF fan, then you simply can't go wrong with Kingdom Hearts - a strange little action RPG that brings both worlds together.
Although it may seem odd, even kiddish, rest assured that it is all in a good way. The amount of nostalgia and entertainment you would be experiencing in this game is tremendous, but of course it all depends on whether or not you are into the Disney characters to begin with, because KH is mostly about Disney than it is about FF.
You play the game as Sora, a kid sucked out of his own world by a strange power of darkness, and you are thrown into an odd universe where you are the chosen one by the Keyblade to close all the doors of darkness. The story is apparent from the get-go, and unfortunately it doesn't develop as much as you would want it to. While it is good as is, the amount of twists is minimal, and whatever twists are present in the story are anticipated and don't do anything except add more drama to a particular scene. So if you're looking into a grand story, you might be disappointed. This is not to say that the story is absurd or kiddish, but rather, it is simple, but on the plus side, it is endearing and amusing and suits the game setting well.
You'd be spending your time going from world to world, sealing the doors from which the Heartless (your enemies) come from. Each world is themed by a Disney movie, from Aladdin's Agrabah to Alice's Wonderland. Each world has its own mini-story, where mostly you would be aiding the main hero of that world to achieve something. For example, in Wonderland, you are required to gather some clues to prove that Alice is innocent and bring the true culprit to the court. All these mini-stories add nothing significantly to the main over-arching story, except the link between all the princesses and something which I am not going to spoil for you. Regardless, they are all fun and entertaining, and whatever story elements from the main plot you receive would be far in between and most probably closer to the end of the game.
That said, the game plays nicely. It is an action RPG, so you get your traditional skills, abilities, leveling up and the sort, but instead of having it turn-based you just button mash you way through. The battles can get hectic and even out of control, especially near the end or when playing any of the Colosseum challenges. On expert mode they are insanely hard, and expect to die a lot if you are not quick-fingered. Some boss battles are ruthless, but with a little patience and some homework, everyone eventually would succumb to your Keyblade. Well... maybe everyone except the Platinum Cup.
Aside from button mashing, you can do combos and string special abilities together, but they all require tapping the same button but at the right time, so it isn't really rewarding (although the damage you inflict is astronomical) and most likely you'd be too busy running about frantically and healing than initiating any combos. You also get incredibly cute summons, but for the most part I didn't use them, and since you can't use them in the Colosseum as well, most likely you'd be forgetting them or using them when you're just fed up with killing so many Heartless and just want to get to point B.
Artistically the game is great. All Disney worlds are rendered in lush detail, although some of them look plainer than others. The animation is superb. All characters are well animated, with no exception, and it is like watching a cartoon come to life. The music is great although it is mostly a short note that loops forever, but the music never gets annoying in any world.
The only possible gripe is that FF characters got a lesser share in the story, and most of them are there for you to battle and defeat in the Colosseum. It is a downside, especially when you see your favorite characters do or add nothing to the experience other than simply existing in the game.
In a nutshell, KH is a great action RPG and well worth the cash, but ONLY if you're into Disney and are not expecting something grand in the lines of other Square games. The story is fun, the gameplay is easy or challenging depending on the difficulty level you select, and overall you will not be disappinted.