You'd be heartless to miss out on this.
Presentation:
The guys at Square Enix are no slouches when it comes to FMVs, and while there aren't many here due to limited space on a DS card, the ones that you do see are look great, and are held up by solid voice acting and good music. The theme song of KHII, "Sanctuary" by Utada Hikaru is found here too, but you won't bash them for recycling old stuff because I can't really think of a better song to capture the mood of the Kingdom Hearts games. The wonderful story is not held back by the visuals, and you really feel for some characters.
Story:
The story is quite excellent and emotional, at times. The game takes place after the events of the first Kingdom Hearts game and follows Roxas during his time in Organization XIII, the second game's antagonists. Some of the members of the Organization appear to be quite interesting, but unfortunately their stories are not fleshed out, to better pace the game. You really get the feeling of how Roxas is struggling with internal conflicts and wondering what his place is in the world. Character development is always good, and while the dialogue won't shatter any ground, it definitely gets the job done and almost enhances the story, by adding simplicity, thus not tarnishing the main plot with pointless dialogue. It's also worth mentioning that the female supporting character is not extremely annoying, which obviously counts for something.
Gameplay:
When 358/2 Days was announced, a lot of people (including me) were skeptical that it would have to be extremely dumbed down to work at all. This, however, is not the case. The KH combat system is still here mostly intact, and fighting looks great, with extremely rare slowdown - again, no small feat on the DS. The only gripe I have is there's usually not more than 3-4 enemies on the screen at once. Remember the 1,000 heartless fight in KHII? Yeah, you won't see anything like that here.
The biggest innovation here is the panel system. They're item slots of sorts where you can insert items to enhance your character. There are many different panels that make your weapon stronger, let you string together larger combos, multiply the amount of magic you can use, and much more. The panel system is surprisingly deep and it's fun to experiment with different builds to find the one you're most comfortable with.
The controls are tight and responsive, and easy to use. The directional pad moves Roxas, the B button makes him jump, the X button lets him browse through his items and spells, and the A button uses whatever item or spell you have selected. You can also move the camera using the touch screen, but it's not recommended as it's a little too "swingy", meaning a very slight gesture will make the camera swing much too fast to see what's going on. I recommend switching to the "Type B" controls, which uses the shoulder buttons to move the camera, which I found much more comfortable. The only finicky control is quickly tapping the right shoulder button twice to lock on to a target. I play on an old Phat DS and the shoulder buttons aren't really as "clicky" as the newer DS models, but that's probably just a matter of taste.
The game is seperated into missions, much like Crisis Core. Some of the missions are short and sweet, while others seem to drag on and are a little too padded. The mission types manage to keep things fresh and not boring, and you'll be conducting recon (basically walking around an area and pressing A whenever Roxas has a question mark over his head), collecting hearts (basically slaughtering every Heartless you see), or sometimes you'll be hunting a particular type of creature. The pace is brisk in most missions and aren't held back by pointless dialogue, which is good.
Conclusion
You'll spend about 25-30 hours on your first playthrough of 358/2 Days, which is a tad short for most RPGs, but that 25-30 hours is expertly paced and not padded like certain other RPGs (I'm looking at you, Final Fantasy). Square Enix has taken everything that's great about Kingdom Hearts and crammed it into a portable form. If you want a quality action-RPG to fit in your pocket, Kingdom Hearts 358/2 Days is very easy to recommend, and it is one of the best games available for DS.