358/2 Days pushes the limits of the DS hardware and delivers an impressive but slightly repetitious game.
In fact this part of the series very rapidly exploded. The first Kingdom Hearts had only four original characters. In contrast I suspect for a lot of people playing Kingdom Hearts 2 there was a culture shock with the plot being not only dependent on having the events of the first game fairly fresh in your mind but the GBA spin off Chain of Memories. If not there were a fair few moments which would be near incomprehensible.
358/2 Days is in some ways a product of Kingdom Hearts 2's excesses in this regard. Set between the first and second game Days takes place between Kingdom Hearts and Kingdom Hearts 2, leading right into the latter game's opening. It follows Roxas' time with Organization XIII. It fills in much of the detail but it also manages to avoid the worst of Kingdom Hearts 2's plot dependence and is easy to follow on its own without missing out too much.
Perhaps the most remarkable thing about Days is the game itself. Days' gameplay is surprisingly similar to the PS2 games with characters and environments which are much closer in quality than I would have imagined possible on the DS. The performance is steady giving a solid frame rate. I'm confident in saying that Days is by far the most technically impressive DS game I have seen.
Gameplay has a few kinks, targeting can be a bit iffy, landing attacks is sometimes a bit arbitrary. Actually hitting with spells becomes a problem which is a problem since they are a fairly limited resource. Other than that the gameplay is mostly solid delivering the slick action/RPG experience you can expect from Kingdom Hearts.
A highpoint for the game is the panel system, it's unique advancement mechanic. Everything from levels, spells, items, accessories, abilities and weapons are panels. You have a limited number of slots in which to put them but slots unlock at a steady rate as you progress through the game. Later you get panels which take up multiple slots improved version of abilities or panels which let you multiply the effect of level panels or multiple uses of spells. These multi-slot panels can be quite awkward shapes so you have to arrange your panels carefully to get the most out of available slots. While the system comes dangerously close to needless busy work the it forces you to consider trade offs of more spells, levels or abilities. Well packed blocks of panels can make your life a lot easier as well as being strangely satisfying to arrange.
The cost of squeezing so much into a DS game is heavy reuse of its environments (most of which are based on environments from KH1&2). The game uses a mission based structure which has you running through the same environments with different configurations of enemies and different objectives. To stretch it's life span out even longer you can revisit past missions which is sometimes necessary to complete optional objectives or reach some treasure as abilities acquired later in the game are needed. You can also play challenge version of some missions which have powered up enemies and special criteria such as completing a mission quickly or taking few hits, completing these can unlock bonuses. In addition to all that there is a multiplayer mode which lets you link with another DS and play as other Organization members (and a few other bonus characters) to team up and play missions. You can also play this mode solo to use the other characters. All in all days has a decent lifespan with my save clocking in at about 50 hours and I did not do all that much of the side challenges. However in that life span Days quite often crossed the line of excessive repetition, to have reduced that at the expense of shorter play time would not have been an unwelcome trade off.
The overall difficulty is quite low even playing on the hard mode, the "penalty" for dying is quite often beneficial. The exception is a few boss battles. They have a fair bit of health and quite often the very specific method you need to beat them is far from obvious and dependent on a lot of trial and error or just looking up a strategy.
While the story to some extent stands on its own some familiarity with the series will help a lot. However it is fans of the series that will get the most out of it. On its own several plot threads are left unresolved, though these are fairly minor. The conclusion to the story is quite unsatisfying without Kingdom Hearts 2 to continue it. For the fans who have followed the series though that won't be a problems and there are some big pay offs. The last mission in particular but I won't say more to avoid spoiling it.
Kingdom Hearts: 358/2 Days is a technically impressive and fun game. Unsurprisingly this is a game series fans will likely enjoy the most but there is something here for everyone.