Pros
Great pay, great job security, very good vacation time. A lot of pros and cons depend on where you are, as the UN is a vast and diverse place. There is often the opportunity to travel, meet new people and learn new things. Depending on your role or where you are, you might even have the opportunity to do good, meaningful work.
Cons
From middle- to upper management selections are all political (usually unofficially so) and don't have much to do with qualifications. Besides limiting opportunities to advancement, this also means that your boss will often be much less intelligent and qualified than you are. People care much more about protecting their position and image than about doing quality work or about serving Member States. One aspect of this is that people just hire their friends as consultants, and consultants in the UN are usually not very qualified, in my experience. Working here for a few years is fine but after that, either become a drone or burnt out.