Explorers of Sky is the definitive Pokemon Mystery Dungeon experience.
Explorers of Sky is the third installment in the Pokemon Mystery Dungeon series, although it might be more appropriate to call it Pokemon Mystery Dungeon 2.5 - its relation to Explorers of Time and Explorers of Darkness is akin to the relation of Pokemon Platinum to Pokemon Diamond and Pokemon Pearl, in that it's basically the same game, only with extra stuff added. I will speak both about the main game and about the extra stuff, so those who have already played Explorers of Time or Explorers of Darkness may want to skip ahead a little, as the question of whether one wishes to additionally get this game if one already owns those effectively hinges on those items.
The game itself is a fairly standard dungeon crawler, in terms of its gameplay. Anyone who has played a game in which they traversed randomly-generated dungeons will know the deal: there exist dungeons whose floors are randomly generated (though using an algorithm that ensures they are always traversible), and in these floors one can find enemies and randomly placed items. The object in each dungeon, depending on where you're at and what has led you there, is either to reach the top/bottom floor, or to reach a certain floor and then do something on that floor (defeat an enemy, find a Pokemon or item, etc.).
The levelling and ability system is very standard Pokemon fare: you can progress from levels 1 to 100 (though your starter Pokemon begin at level 5, not 1), and you can have up to four moves, each with their own levels of PP that control how many times you can use the moves. There are no Pokemon Centers, however - HP and PP are both restored to 100% whenever one either leaves a dungeon or reaches a midway checkpoint.
As is the standard in dungeon crawlers, everything is turn-based - you move, then your partners move, then your enemies move. Most of the moves are exactly as they appear in the standard Pokemon games, although some have some twists on them (for example, Discharge attacks every Pokemon in the current room, rather than only the Pokemon immediately in front). Each Pokemon can also, as in the main RPGs, be equipped with one item that either boosts a stat or bestows some other ability (resistance to poison, for example). And, as one has come to expect from dungeon crawlers, there is a penalty for losing all your HP and fainting in a dungeon - in this case, you lose half of your money and half of the items in your inventory. There exist a bank and a storage facility at which money and items can be protected from forfeiture, and any player will almost assuredly make ample use of these places.
Those who are familiar with dungeon crawlers will know not to expect a story that is too in-depth, but oddly enough I must report that the story in Explorers of Sky is actually perhaps the most mature story that has been seen to date in a Pokemon game. The narrative is still very much G-rated and kid-friendly, but at the same time it was surprisingly satisfying.
To be brief, the gist of the story is that you are a human who has been turned into a Pokemon, and you are discovered unconscious on the beach by another Pokemon who becomes your partner in an exploration team. As the story unfolds, you learn that someone is sinisterly stealing items known as Time Gears, which are required for the proper maintenance of time on the planet, and you must, in addition to your exploration duties, track down this individual.
Although I have no doubt that those accustomed to epic, realistic stories will find the story in Explorers of Sky laughable, I actually found it to be very well done - it actually contained, in my view, quite legitimate plot twists, intrigue, and even glimmerings of sacrifice, loss, and love. By the end of the game, I honestly cared about the characters, and I felt sad or glad when they felt the same, which is the metric I always use to determine how good a story is.
As far as value goes, Explorers of Sky is very faithful Pokemon fare, in that there is a large wealth of that which can be done outside of the main plot. In addition to missions in dungeons that actively advance the plot, one can also obtain side missions either through the billboards at the explorers' guild or through entering a code at the title screen, and these provide the player either with extra money, items, or Pokemon recruits. Pokemon can also be recruited at random when you defeat them in dungeons, and you can form a team of up to four Pokemon before embarking to a dungeon (including you and your partner, although later in the game you can optionally form a team without you or your partner). There are also special missions once you've completed the main game that allow you to recruit legendary Pokemon, as in the standard RPGs, as well as ones that take you to very difficult dungeons.
The soundtrack in the game is perhaps one of its most understated positive qualities, as well - it does not generally assert itself too strongly, but after listening to enough of them, I've come to the conclusion that the game, in fact, has in my opinion one of the best soundtracks I've heard to date on a DS game. The tracks are very well-put together, and are often genuinely moody in nature; they are not your standard fairy tale "la la la everything is happy" type of songs. I even found myself actively seeking them out for recreational listening after I beat the game. A great soundtrack obviously does not make a game, but in this case I felt that it was a great addition to an already great package, one that certainly did not need high-quality music to sell.
As for the extras in Explorers of Sky above and beyond Explorers of Time and Explorers of Darkness, probably the largest addition is the existence of five special episodes, as they're called, that star different characters from the game (for example, other members of the explorers' guild), and generally just serve to provide interesting backstory or added depth for the other characters in the game. While I felt that the first one was kind of silly and not terribly gripping, the other four I felt did materially add to the game. There are also some new areas in the main game, such as a juice bar, a place where you can recycle items you don't want for items you do want, and a mountain that you can climb to get a jukebox that enables you to play all the songs in the game.
So now we come to the bottom line: should you buy this game? If you hate Pokemon and find dungeon crawlers tedious and boring, then the answer is no - there is probably very little you will find to enjoy here. If you like either Pokemon or dungeon crawlers, then it could very well be a solid addition to your catalog, either way. If you love both, then it's definitely a buy if you're at all interested. It's just a downright great Pokemon game and great dungeon crawler, for those who are interested in such a thing.
And if you already own Explorers of Time or Explorers of Darkness, then this question depends on whether you want to play it again and additionally play some special episodes and see some new sights. If you do, then pick it up; if not, then the game you already own is probably sufficient.