Pros
Benefits are amazing, Salaries are really high considering the nature of work. Sick days 30 days, but could be unlimited if you have chronic conditions. Home leave policy - meaning the company pays for your trip back home if you are international professional staff (P staff category), and G staff - General Service category does not have such entitlements. Education grant - meaning college tuition is covered up to 75% for each kid as far as I know (only for P staff). If you are on travel pretty much everything is paid and taken care of. There is even a rental subsidy for staff (P) which covers a portion of your rent.
Cons
Very little room for growth as senior positions are held until retirement. Many people get stuck because of the benefits they get. Pretty much all higher positions are held by nationals from large donor countries. You will never work with children (if that is your passion) unless you go to the field, even there you are not in direct connection with the children but Governments. Many people suffer from high stress and PTSD due to working conditions (especially in the field). People are placed in positions that they are not qualified for. Executive management is appointed by the Governments not selected based on leadership or experience. Executive level is held by USA always. In general, they are trying to make good but it's a piece of one huge machinery.