I first ordered this game not knowing quite what to expect, but when I started playing it was nothing like the original first two games in the series. Sure the basic plot for each world, (game plot totally new btw), is roughly the same. The way you play it out is far different to methods seen in the past.
The new battle system will have you rethinking how you play Kingdom Hearts because you have to be more rigorous with your choice of attacks as they are quite limited as to what you can use at one time. The AI finally has some sense with the enemies dodging various attacks, leading to a real battle instead of just a slaughter. Also I noticed enemies have weaknesses to certain elements, which only deepens the strategy aspect.
The battle itself and the amount of options you have during battle will have you coming up with completely new strategies unheard of in Kingdom Hearts; which is a nice refresher.
The average world is smaller then what has been expected out of the last two games, but this is only a minor problem as they are packed with adventure, enemies and the occasional mini game (which are quite enjoyable in themselves). New worlds, never before seen in KH is a lovely site bringing new characters and worlds into the mix.
Birth by Sleep (although I have yet to complete it) has been a rollercoaster of a game and to top it all off, along the way this game may just answer some questions you've had about KH for several years since KH2.I believe this game fills in a bit more of the jigsaw puzzle of a plot that is Kingdom Hearts.
It's a must buy for any KH fan but to all those who have yet to experience the epic Kingdom Hearts series I highly recommend playing the first two games in the series first, just so your brought up to speed on the whole concept.