Kingdom Hearts DS is definitely worth playing for its story and for its fun, altough repetitive, gameplay.
Regardless, I had faith in 358/2 days, because it stuck more to the series's roots and it basically gave what Kingdom Hearts fans wanted in the first place: a touching story along with the ass-kicking of Heartless.
First off, let me just name off some of the positive aspects: to start with, the story. It is slow at first, but once the plot increases in momentum, the story can become very involving and affecting, especially if you played the previous games beforehand.
The game is also the best looking handheld game on the market. Characters and environments are represented so well that they can practically be compared to the PS2 originals from which they are based. It really does look beautiful and it is really shocking to contemplate a game surpassing this game in terms of visuals.
And the final major positive note is the panel system, which is basically a grid system used in gameplay that represents each spell, ability, item, even each level you have. This systems give a much better sense of character customization that the originals did not have. A lot of the time, because of the game's harder difficulty, you will have to strategize what abilities and items should occupy each panel, and it gives a sense of depth to the game, which it desperately needs.
As for the negatives, they are sadly apparent in a handful of crucial areas. To being with...well, the story. While I did complement it for how touching it can be, there is just too much filler and the dialogue sometimes ranges from lacking (I can't count the number of times a character has "..." as their final line of dialogue) to overly angsty (Again, I cannot count how many times characters in this game complain about not having hearts).
The gameplay retains the spirit of the previous games, and for the most part it does well in converting the original PS2 experience to the DS. However, over the course of the game the missions tend to really drag on and get very repetitive. True, in KH1 and KH2 the gameplay was sometimes repetitive, but at least the monotony was broken up with cutscenes with Sora and the various Disney characters that were in those games. In 358/2 days, 98.9% of the dialogue is within the boundaries of Organization Xlll, and while that is fitting for what the game is about, that still kind of trivializes the point of going to Disney worlds to being with.
And finally, the soundtrack. Mostly ripped from KH2, it still sounds great three years later, even through DS speakers. Sadly, I can't say as much for the newly composed music, which is mostly generic, save for the Neverland theme.
Is Kingdom Hearts worth playing if you are not a fan of Kingdom Hearts? I am barely confident enough in this regard to say yes, it is. However, if you ARE a fan, you owe it to yourself to play it. If you are willing to tolerate its repetition, it is a 30 hour treat treat to the eyes, ears, and heart, as well.