Kingdom Hearts: Birth by Sleep is the latest installment in the Kingdom Hearts franchise. Those who love the series will definitely enjoy this and those who are new to it can still appreciate what's there.
The game takes place 11 years before the events of Kingdom Hearts(1) and focuses around the story of 3 keyblade wielders. Terra, Ven, and Aqua. They all have a distinct personality and great voice acting to go along with it. However, there are a few really bad voices that come along but are usually just for minor character(ex. Snow White).
The battle system is the best of the series. It's no longer boring to grind because you're always switching through different skillsets and trying to unlock new combinations and abilities. The melding and command deck system is a little overwhelming at first but as you play you will become accustomed to it and eventually master it. It's quite an elegant progressive system.
D-Linking, although seeming useless in the normal mode, is essential in the proud, and critical modes. This alows you to take on the skills and buffers of a connected character such as the characters you meet in other worlds. Also, when you D-Link your character regenerates all of his/her health.
The story, unlike most Kingdom Hearts games, is simple and surprisingly not melodramatic and it really benefits from it. It's unfortunate though that you don't get to meet as many Final Fantasy characters in this installment. However, the game doesn't really suffer from it(and Zack is a much better character than Cloud).
Graphics are pretty good. Better than the first KH but not as good as the second. Though the mouth syncing is much better than both and there is less dialog boxes and more actual talking. The game's framerate stutters in some portions and every now and then you'll get some freaking out of the camera but it's not so often that it's intolerable.
I'd say now, without a doubt, that Kingdom Hearts: Birth by Sleep is the best in the series. It's been a great year for the PSP. This game had a lot of competition to live up to but it definitely pulled through. The game has many set pieces and great plot points. This, accompanied by a great battle system and graphics, make Birth by Sleep a heavy hitter.