Pros
If you are hired as an official, the salary and the benefits are quite good. As you would expect, the environment is very international. It also actively promotes gender equality and is quite friendly towards non-heterosexuals. If you have a certain level of expertise and drive, you can expect to work on extremely interesting issues that have an impact on the lives of hundreds of millions of people.
Cons
Decision processes are extremely slow and opaque. Working methods (IT specifically) are embarrassingly antiquated and inefficient (unfortunately because they are often decided and implemented by people who would not go very far in the private sector). It is often extremely frustrating to be reporting to individuals who have neither subject matter competence nor management skills. Professional development is pretty much left to one's personal initiative, to the effect that one runs the risk of quickly becoming unemployable elsewhere.