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.
Deep in the Hall of Collosia, there is a Mad Monk that, while visually identical to other Mad Monks, has 1000 hit points, and will drop the Bronze Spear when defeated.