The Mystery Dungeon series of pokemon, in my oppinion, is the weakest of the bunch.

Mystery Dungeon is a role-playing game, and it revolves mainly around the player becoming a pokemon, which is chosen for them based on their answers to a questionnaire at the beginning of a new game. While this is quite exciting, the fun wears off when you discover that no matter how hard you try, the result will always turn out as one of the same group of pokemon. The pokemon you get assigned to be does not have an effect on the events that will happen during the game.

During gameplay, the player will have to complete "jobs" to rescue pokemon. These take place in one of the 69 (8 have been added in Explorers of Sky) "randomly generated" dungeons.
The dungeons are quite a dissappointment, as the player basically just has to find the "stairs" and get to the floor that the trapped pokemon is on.
The dungeons can become repetitive, and hard to tell the difference from one to the other. The graphics, however, do make the gameplay more interesting as attention to detail has been taken on these aspects of the game, and environments are nicely presented.

Pokemon games are mailny associated with battles, and in the main series' such as Pokemon Diamond and Pokemon HeartGold, the battles are getting better and better with every new release. The Mystery Dungeon series greatly lets the side down here, as battles are nothing more than a turn-taking thumpathon between enemy and player, with the odd special attack thrown in if you get lucky. This quickly becomes monotonous, especially with "stronger" pokemon, as you may be thumping for ages before you win (or loose).

Difficulty is minimal, which is surprising for a game so lengthy. Frankly, it would be hard to find any aspect of the game challenging for an enthusiastic gamer.

So, overall, the mystery dungeon series of pokemon is a bit of a let down compared with the other Pokemon series'. It plays like a basic, repetitive game, and with only a few new additions in Explorers of Sky, and no change to the plot, I wouldnt spend the money to experience the same game again.