Way better than GameSpot says
The game revolves around three characters, Terra, Ventus, and Aqua, each of which have their own separate storylines that you get to play in this game.
-PROS-
Kingdom Hearts: Birth by Sleep is an "action-RPG" type game, so fighting is primarily the main fun you will get out of this game. The fighting is highly customizable, and each of the three characters have different fighting styles. There is a ton of variety in the fighting system in KH BBS, from melding your existing commands to form more powerful, elaborate ones, and creating abilities in the process.
The story in this game reveals many mysteries untold in the previous Kingdom Hearts video games, from new beginnings and the origins of many of the main events in the future Kingdom Hearts stories. However, I suggest you play each game by order of release date to fully immerse yourself in the Kingdom Hearts story.
The length of the game is well-worth $30. With three different campaigns, my total playing time was 50 hours with this game on "Proud Mode". There are also treasure chests, abilities, and stickers that will keep you occupied with this game for longer after you finish it.
There are also many side-track activities in this game, including the Command Board, which you use to level up and obtain new commands while playing a board-type game. The is also the Mirage Arena, which is also used for ad-hoc play. The Mirage Arena has different rounds of enemies to defeat, aiding in sharpening your fighting skills and leveling up your character, and extra bonus prizes. Upon completing the main campaign(s), there are also extra bosses to complete, which are a fulfilling challenge that will also keep you glued to this game.
-CONS-
There were no cons regarding the story or presentation of this game, but as I said before, I suggest you play each Kingdom Hearts game in order of release date if you are new to the series. Do not start your Kingdom Hearts experience with this game. However, the game does have some hardware issues.
A major blowback with this game is loading times. Even after installing 600MB of data, loading took at least 15-20 seconds, and I cannot imagine how long without data installed. It also is a hindrance that loading occurs so frequently, like inbetween major cutscenes. This was very bothersome, and the loud disc spinning in the PSP didn't help that either (but that isn't the games' fault).
The game is very vibrant and colorful, but there are frequent screen flickers and frame rate drops. Many times during battles, the game will momentarily freeze for a split second. Also, during cutscenes, the screen flickers, but not in a manner that is extremely noticeable.
-SUMMARY-
The game is a masterpiece in regards to gameplay and storyline. However, it seems that maybe the PSP can't handle the opulence of this game, or I'm just being bias. If you can put the external issues aside, it is definitely worth playing if you are a follower of the Kingdom Hearts series, but it is not for those who are unfamiliar with the story.