I've played all of the Kingdom Hearts video game as of present, including the Japanese Mobile "Coded" Spin off title not yet released in the US or UK, and this could very well be the best out of the lot.
You are introduced to three new characters; Ventus, Aqua, and Terra, three friends set on a journey to become Keyblade Masters. Some people may be put off by the fact that Sora and Co. May not be the main focus, but this honestly does not matter as the character development and voice acting throughout the game is on par with the previous console versions of Kingdom Hearts.
Graphically for the PSP, the game is stunning, and can nearly be described as being as good as the PS2 itself.
Although I do sometimes miss playing as Sora with Donald and Goofy, a lot of fun can come from playing as the seperate characters, and you find out early on that they will all play a part in Birth By Sleep.
This game is not one to miss. Buy it!