Although the card system is a pain to get used to and the slights are badly explained at best, this unique KH game is quite in depth, even for it's series, being the first to include a second storyline with another character to play, although this is a much simplified version. 

Sora can use pretty much anything to attack, a fully customizable deck to use and approve of and whatever slights therefore you can imagine that you have the cards for, once you finally figure out how the system works.  Many of the later bosses do require these high end slights though, and if you aren't prepared you WILL die constantly, which is quite frustrating for a player, who may use any combo of low slights and never win against a faster opponent, eventually running out of cards, and therefore commands, making your poor Sora helpless and waiting for death.  Also, healing is quite hard to pull off for no reason, with many of the later bosses targeting your heal cards and breaking them so you can't use them for the rest of the battle.  Therefore, the endgame is quite hard for the first time player to pull off without a good guide, which is a definite negative against the game in my opinion. 

However, if you do manage to kill Marluxia and avoid his devastating attacks, you unlock the second storyline, which fills in the holes and after story between KH1 and KH2.  I'd actually played KH2 before finding this game, so this really did fill in a lot of plot holes for me, like the note which only included "Thank Namine" that Sora reads early on.  I'd just thought that something weird had happened or that Sora hadn't properly reunited with his heart from the end of KH1 and that they needed him to sleep so they could fix that while his nobody, Roxas, was just running around aimlessly. 

The second storyline starts about halfway through Sora's story and goes a bit after his ends and ends up sleeping, and stars Riku and King Mickey.  Riku fights with a predetermined deck, which does save the player a ton of time with the customization, but the changing decks dependant on the worlds are often not very good *cough* Wonderland *cough* and if they are decent, like with Halloween Town's, the heartless reflect it, making the rank and file enemies much more annoying to deal with.  Grinding is therefore that much more annoying, and most of the storyline is gone due to Riku's rejection of both light and darkness, which although it fits, I'd like a bit more really...  Riku can go into "Dark Mode" where he does a ton more damage and has his attacks faster? But doesn't last long if you haven't leveled up your Dark Points (DP).  Riku can also level up HP at the same rate as Sora, but needs more since none of his decks include healing cards, although they don't need charging to reshuffle, so he relies much more on boss enemy cards to heal, especially the Oggie Boogie Card, which heals you passively 10 times for about 20 HP each time?  His only other option is the King Mickey cards, which are his only friend cards ;( and rather rare compared to Goofy and Donald's from Sora's story.  However, Riku will hit much harder from the same cards due to you able to level up his Attack Points (AP) from leveling, about once every 4 levels, and this is always a priority, since he is basically a glass cannon.  Dueling, similar to the high leveled slights from Sora's story, is pretty much unofficially required at the end bosses, and triggered by hitting an opponent's card with one of the same value.  You then must break all of their cards before performing a slight of the cards which broke the opponent's within the time limit if you were successful.  This is quite useful and makes end bosses so much easier, but does burn up your cards quickly, so be wary.

All in all, build up your deck well as Sora, and plan your slights well with your predetermined deck as Riku and you should succeed
A very entertaining game, and possibly one I'll play again in the future for it's wonderfully in-depth story line.