Pros
Great mission. Smart people who really care about development and the work they do. Interesting work for the most part (especially in the field versus h.q.). UN has great benefits package if you are a full time staffer. Strong positive external recognition of organization.
Cons
HQ is very bureaucratic. Decision making and hiring decisions are not very transparent and are not communicated well to staff. Hires great people due to mission/reputation but often fails to develop or support them leading to a lot of turnover. Not very forward thinking when it comes to management and innovation exception a few flagship, highly publicized areas. If you are "in" toucan get a lot of support but if your area of work is "out" of current fashion can be a bit frustrating.