Mad Monks are not especially fast, and can only attack at close range. However, any successful attack from a mad monk may also inflict Confound or Plague on the player, making them potentially more dangerous than Rogues. They have low HP and can easily be killed with charged attacks or alchemy.
Mad Nonks are good for earning jewels quickly, as they offer much more than Rogues, and have far fewer hit points. Players interested in levelling up, however, should focus more on Rogues, as they offer much more experience.
In Crustacia, there is a friendly Mad Monk merchant, which can be spoken to when walking behind the crates to trigger a conversation. He will offer the player an Amulet of Annihilation for 10,000 Jewels. If the player does this, the Monk also gives them a Charm they do not already have, in this order: Chocobo Egg, Magic Gourd and Wizard's Coin. If the player has all of these, he will give 3 Meteorites. The amulet seller does not appear if the player already owns an Amulet of Annihilation. If the player makes the purchase and gets rid of their Amulets, the seller will reappear. This is the only way to farm Meteorites. Go here to read more on getting rid of amulets.