Kingdom Hearts: Birth by Sleep does not disappoint.
STORY:
If you're unfamiliar with the series, the general idea is that you play a hero who wields a weapon known as the Keyblade and you visit various Disney worlds (Aladdin, Hercules, The Lion King, Sleeping Beauty, etc.) to fight the forces of evil. This is a gross simplification of the story, but that's the general idea.
This game takes place about 10 years before the events of the first game. Instead of usual lead Sora, the story revolves around three friends: Terra, Ventus, and Aqua, who are in training to become Keyblade Masters. Early in the game, the heroes get sent on a mission to defeat monsters called "Unversed" that have appeared in the various worlds. In addition, they are tasked with locating the mysterious Master Xehanort, who has vanished under strange circumstances.
Since this is a prequel, we know that things can't end well for our heroes -- otherwise Sora would have nothing to do in the other games. As such, BBS has a much darker story than any of the previous games. The ending in particular is rather heart-wrenching, but I won't spoil any of the details. Despite saying all this, don't expect it to be as dark or depressing as something like the Matrix or Battlestar Galactica -- this is still a Disney game and you will definitely find yourself smiling and/or chuckling to yourself at many points throughout the game.
The story has been very well crafted and all the new characters have a great deal of depth to them. Aqua in particular I found to be a very good hero; I like her as much as, if not more than Sora. The main villain of the game is easily the most cunning and insidious villain to appear in the series. In fact, I would argue that he is one of the best villains to come out of Square-Enix in a very long time.
Do you need to play the other games to appreciate this story? No, but you will appreciate certain events more if you have played the other games (in particular, Chain of Memories and Kingdom Hearts 2). Even if you're new to the series, I'd still suggest picking it up.
Story: 10/10
GRAPHICS:
This game is beautiful. It is likely one of the best looking games on the PSP. It easily surpasses the graphics of Kingdom Hearts: 358/2 Days, and is probably about as good or slightly better than RE:Chain of Memories or Kingdom Hearts 2. In particular, I found myself admiring the detail of the expressions on the characters' faces during cutscenes.
Graphics: 10/10
SOUND:
Voice acting is, as usual for the series, stellar. Perhaps not the best voice acting ever (the Metal Gear series is probably better), but nevertheless quite good. Disney characters tend to sound just like in the movies (in some cases, voiced by the actual Disney voice actor!) and the original characters are voiced well. Of the three leads, Terra is probably the weakest for voice acting, but it's not intolerable. Special mention goes to Leonard Nimoy for his role as Master Xehanort -- he is outstanding at voicing the character.
Sound: 9/10
GAMEPLAY:
If you enjoyed the gameplay of Kingdom Hearts 2, you will likely enjoy playing BBS. The combat is fast paced and there are a variety of attacks to use, from standard keyblade strikes to casting spells to using shotlock attacks to hit groups. You can customize your character's arsenal of attacks by selecting different types of attacks and moves, allowing you to tailor the gameplay to your own preferences.
In addition to just finding new commands/spells as the game progresses, you can also fuse 2 spells together to make a new, more powerful one. In addition, you can choose to add a crystal to give the spell extra powers/abilities. It is like a simpler (albeit less addicting) version of the fusions that occur in the Shin Megami Tensei games.
Summons and Drive Forms from KH2 return as D-Links and Command Styles. D-links allow you to inherit special powers from a friend that you've met in the storyline (like Mickey or Snow White). Command Styles alter your basic attacks. By using various attacks and spells, you charge up the command gauge, and once it is filled, you may enter a Command Style. This greatly improves the effectiveness of your standard Keyblade attacks, just like entering the Drive Forms in KH2. By charging the command gauge with different types of attacks, you may enter different Command Styles.
You play through the story 3 times: once as Terra, once as Ventus, and once as Aqua. At first, you might expect this game to suffer from a lot of repetitiveness, but it never feels like that. While you visit the same worlds, you have different interactions with the characters and see a different part of the storyline. You may not even visit the same parts of a particular world. For example, in the Cinderella world, the Ventus part takes place completely within Cinderella's home, while Terra never even goes inside.
After playing through all 3 stories, if you have achieved a certain condition, then you get to play though one final story as Aqua. While it's pretty short, it gives you an extra boss fight and provides closure to the story.
As usual, there are plenty of fun minigames to play, and a couple not-so-fun minigames. But they add a nice sense of variety to the standard gameplay. Multiplayer is hypothetically fun, but only if you know someone else with a PSP. As I do not, I can't comment on that.
There are 4 difficulties to the game, as opposed to the usual 3 for Kingdom Hearts games. The newly added "Critical Mode" makes the game a serious challenge. Unless you do a lot of level grinding, expect the boss fights to be on par with Devil May Cry or Ninja Gaiden for difficulty. So if you're inexperienced, do not choose Critical. If you're up for the challenge though, it is a very fun and rewarding experience.
Gameplay: 10/10
At the end of the day, this is my favorite KH game. No question about it. If you're a fan of the series, then you're probably going to pick this up regardless of what I say. If you're unsure, I'd say go for it.
One last note: If you buy the game, I STRONGLY encourage you to do the full 600 MB data install. Unlike some other games, this one really benefits from it. Also, if you only have a PSPGo, you may be out of luck; at time of writing this game is not available for download from the PlayStation Store.