Warning: VERY long
Hello, my friends.
I am going to tell you about this great game I bought 2 weeks ago, which is awesome.

Where should I start? Oh!

Main Storyline

When you start the game, you get to see all the Organization's members, known as “Nobodies, with a Capital N.” Nobodies are people who came into the world without 1 precious thing, a heart.
The Organization's main goal is... Guess what? Kingdom Hearts. Kingdom Hearts is the goal of the Organization. It's the thing that gives Nobodies what they lack; a heart.
The Organization consists of 13 members. No. I: Xemnas; No. II: Xigbar; No. III: Xaldin; No: IV: Vexen; No. V: Lexaeus; No. VI: Zexion; No. VII Saïx; No: VIII: Axel; No. IX: Demyx; No. X: Luxord; No. XI: Marluxia; No. XII: Larxene; No. XIII: Roxas.
If you hve noticed something, all their names have an X in them. Coinsedence?
And, each one has his/her own weapon. For example, Xemnas uses Etheral Blades while Marluxia uses Scythe.
You first start at the Castle That Never Was.
You talk to Axel, then you go to the Round Room (The room you saw in the first cutscene ever).
You'll meet the new member of the Organization, No. XIV: Xion.
Xion is a puppet that was made to copy Roxas's ability to wield the Keyblade, which is the only thing that can complete Kingdom Hearts. They want to copy him so they can complete Kingdom Hearts faster, but something turns wrong. The puppet now has its own identity, which turns things bad (i.e.: Roxas's ability to wield the keyblade goes down.).
You'll find the rest when you play it. ;D
So, I rate the Storyline 9/10, because you can only play with Roxas (But rarely, you get to see and play with Sora).

Gameplay

The gameplay is pretty awesome. You have panels. But fiiiirst, what are panels?
The panels are some 'panels' that you can place in Panel Slots to get different effects.
There are Item Panels(You can only use it once), Support Panels(Level Ups, Backpacks), Magic Panels(Panels that do anything with Magic, of course), Ability Panels(This is where you could take panels to power-up your skills. You can glide/roll/block/etc. with certain panels) Weapon Panels(Ahh, this is the tab where you could select Weapons from it. They're not technically weapons; they're gears. Each gear upgrades your Keyblade.), Ring Panels(Installing one of those may boost your stats, grant new abilities, or provide protection from certain elements), and finally, Synthesis Materials(You can't install these. Instead, take them to the Moogle Shop(Main shop of the game and the only one), where you can use them to make new panels.).
And, of course, you could play more than one character, but sadly, only in Mission Mode. ;-;
There are worlds in the game. You'll figure them out later.
I give it 9.9/10.

Cutscenes

The cutscenes are pretty awesome. Actually, best of all the DS games I've played.
The cutscenes have sounds of them talking, but not all the cutscenes. After you finish the game, you get to see every single cutscene you've seen.
I give it 10/10.

Graphics

When I saw the graphics, I was pretty amazed. They're more awesome than Pokemon HeartGold/SoulSilver's graphics. Pretty awesome. They're not THAT good, but you can still say they're good.
I give it 10/10. :3

Sounds

When I first heard the Main Menu's song, I just wanted to listen mooore and more. It was plain epic.
Every world has its own music; Boss music, Normal Music, Battle Music. And, each weapon has its own sounds(I guess), and most of the characters do have their own sounds. Even the enemies have their own sounds.
I give it 10/10

Overall:10/10

This game is a total MUST buy.