What exactly is the attraction of this franchise? It is my understanding that, beneath the RPG elements, it's basically a drawn out hack-and-slasher with an absurdly convoluted story (Heartless? Nobodies? What?) and questionable controls. I don't own any of them, I've only sunk a couple of hours into a friend's copy of Kingdom Hearts II, and it appears to be riding on the appeal of the Disney licence heavily.
Unfortunately, I fear that Epic Mickey may be doing the same, which for me personally is not enough to keep me playing.
The first one, at least, did ride off of nostalgia. A stronger focus was placed on the Disney and Final Fantasy characters. It was after Chain of Memories that the focus was instead placed on the original story and characters.
I can only speak for myself, but I will admit that I got into the series because of the Disney and Final Fantasy references, but I stuck to the series because despite the story being convoluted it's enjoyable. I really like the characters, especially Sora may be one of the best non-Final Fantasy characters Square has created.
The gameplay is indeed a hack and slash, but it has been refined so that it's more than just a Samurai Warriors clone. I really liked the battle system in Kingdom Hearts 2. It was far more polished than the first game and there is a lot of attacks and skills you can pull off in battle. Plus, there's an odd sense of satisfaction when you kill off hundreds of enemies in just a few swipes.
It definitely ain't for everybody since gameplay CAN get redundant and the story hard to follow. But in my case, it is very enjoyable.