The first thing you'll need to get your head around is the Command Deck system. In many ways, this is just like the immortal materia system from Final Fantasy VII; you equip an ability (known as a command) and the more you use it, the more powerful it will become. Your deck is simply the selection of commands at your disposal, and can be edited as you wish. Commands can be found in chests and bought from shops along the way, and if you find one you're particularly fond of, you can swap it into your deck. Improving and editing your deck is surprisingly addictive, and you'll find yourself spending more and more time in the menus as the game progresses.
Another interesting addition to combat is the brand spanking new D-Link system. After becoming acquainted with certain characters in the game (which in Snow White's case, for example, involves saving her ass from man-eating trees), your hero is able to form a link with them in combat. This enables your hero to use the abilities dictated by that character's command deck. So, if your character doesn't happen to know any healing spells, you can use D-Link to use the abilities of a character that does. Pretty handy I think you'll agree.
By stringing together attacks of the right nature, you can also change your combat style. Throw a fire spell into the mix at the right time, for example, and you might activate the FireStorm style, which imbues your blade with that particular element. Experimenting with different command decks and working out which abilities provoke which command styles always manages to keep combat feeling fresh. In conjunction with D-Link and the option to combine two commands to create combination attacks, Birth by Sleep has one of the deepest real-time battle systems around.
Combat is great then, but it's hard to remain quite so complimentary about the narrative. While the story might make sense to those who've graduated with an A at Kingdom Hearts College, everybody else might find the plot slightly overwhelming. Just like Kingdom Hearts 2, it's somewhat convoluted, which I've always found quite jarring alongside the simplicity of the Disney plots. It's still digestible, though, and on the whole the voice acting is very good. The vocal talents of a certain Leonard Nimoy (Xehanort) are sure to delight many, and some of the original Disney actors have been brought in to really bring their characters to life.
It's been a good year for the PSP, especially for quirky RPG affairs such as this. I was ready to dismiss Birth by Sleep as being just another spin-off; a quick cash in on the Kingdom Hearts name, but I'm happy to have been proved wrong. In many ways, it actually outshines its console counterparts. Bar a few niggling control issues, the battle system could well be the best in the series yet, and at times I actually felt it looked better than its PS2 cousins. For those still enduring the long and painful road to KH3, Birth by Sleep is a fantastic stop along the way.