Monks are excellent for just about everything, except the exp cap makes killing Demi-Lichs just a bit difficult without finding a nice weapon. Additionally you may want someone who can hold a shield (i don't think monks can) when going up against Beholders (ie shield of Balduran, i cleared out the Beholder City using just Minsc this way, he didn't even have to attack
). As a monk the only thing you have to watch out for are instant removal spells such as Desintegrate, Petrify, Imprisonment and that nasty ability of Mind Flayers.
Monks ==> Powerhouse