The idea is interesting and the thought of being a Pokémon instead of training one is a fun twist.
The game kicks off with a personality test which asks you a number of silly questions ("a human hand comes out of your toilet! What do you do?"). These somehow determine which sort Pokémon you are, and therefore which one you'll be controlling in the game (by choosing to shake hands with the hand in the toilet, we became Psyduck. Fair enough).
The story goes that you've woken up as a Pokémon. Before you can figure out what's going on you meet another Pokémon (your choice of eight or so) who becomes your partner and decides to set up a rescue team to help other Pokémon. This involves travelling through dungeons until you get to a certain floor, then finding the distressed creature and helping it out, either by touching it and warping it out of there, or giving it the item it needs. And that's about it.
Mystery Dungeon's main gameplay feature is the fact the dungeons randomly generate every time you enter them (hence 'Mystery Dungeon' we suppose), which means if you leave and enter again it'll be a different maze. You and your partner will have to walk through the maze until you find the staircase to take you to the next level, all the while randomly encountering wild Pokémon who'll want a fight.
Battles are sort of turn-based with all the action happening on-screen instead of cutting to a separate battle screen. Fights are frustrating though, mostly due to the poor intelligence of your partner who'll pick his own moves and, in later dungeons, tend to exhaust his special moves before you reach the harder levels.
After you've given a wild Pokémon a kicking, your defeated enemy may ask to join your team, meaning it's not uncommon for three or four Pokémon to wander about as a group. This makes battles even more annoying, since they'll tend to be a case of you facing and fighting your opponent with your three buddies standing behind you, doing nothing because you're in the way.
This is the sort of game that screams out for a two-player co-op mode, but no such luck. Instead, you're stuck with your thick-as-mince computer-controlled mates who are more of a hindrance than a help. Add to this the fact that the action's extremely repetitive with countless missions asking you to perform identical tasks, as well as useless stylus controls that will have you using the D-pad within five minutes, and you have a game that quickly becomes a chore, even for some die-hard Poké-fans.