This game is remarkable. The story ties together everything and answers many questions that have been left up in the air in the series. The story is told in 3 parts from 3 different perspectives. You can choose between Ventus, Terra and Aqua. Terra has strength but he's slow and average magic. Ventus is quick, and average in both magic and strength. Aqua is strong in magic, average in speed and weak in strength.
I recommend playing through in this order for the story. Terra, Ven and then Aqua. Once you finish a story you'll be opted to save a clear file. Keep them so when you have all 3 of them as well as all the Xehanort Reports you can play the Final Chapter. It's well worth it.
With all the data saved you can view the time lines of each of the stories to help you figure out who is where and when. Each of the 3 characters go to every world, but at different times. They sometimes run into each other, sometimes they just mess each other. The way it all unfolds is masterful. It's easily one of the best stories in KH, rivaling KH 358/2 Days for me [which is my personal favorite story in KH so far].
Gameplay is the best in the series. It's a mixture of everything done so far. There's also a healthy amount of platforming in this game, which is nice. You can meld abilities together for more powerful ones. There are minigames, there's online via adhoc mode [with the PS3]. The racing minigame is awesome. The rhythm one is okay. The Fruitball [volleyball meets soccer] is kind of lame ot me. There's also the Command board which is like a board game. You roll a die and then you move along a board. Gain points to spend buying spaces which will cost the CPU to have to pay you. [like Monopoly] This allows you to spend points to level up ingame abilities as well as earn new abilities.
There's the battle arena as well. Which is probably the best, it basically takes the place of the Olympus Colosseum Cups. Since they're absent from this game. Despite visiting the world.
Which brings me to this, there are a ton of new worlds. Sleeping Beauty, Lilo and Stitch, Cinderella, the original world Keyblade Graveyard and others. All of which are great. There's tons of content in this game and I can't recommend it enough. It's worth buying a PSP for.
If you're new to the series then a lot of things will probably not click with you. Since a lot of the revelations explain things in the rest of the series. It's best to play the games in their order of release I think. Exception being KH: 358/2 Days which can be played after Chain of Memories, since it's story starts around the start of that game. KH, KHRe:CoM, KH Days, KH2, KH:BBS.
Graphics are amazing. You have the option to install data to make load times faster and there are options in the menu to allow you to up the graphics. Install the biggest size possible for best performance and turn up the CPU option.