Pros
Excellent opportunities to live and work around the world and experience different cultures. I had the unique chance of living 2 years and a half in Ethiopia and travelling all over Africa and then 2 years in Beirut, where I learnt invaluable lessons about the Middle East. Really expands your horizons. Salary is excellent. I had the opportunity to publish papers and book articles in prestigious journals. My work was extremely interesting, especially in Africa, and I could feel the impact of the research we were producing. Working directly with member states in the countries can be very rewarding as you get to know what the problems are on the ground and challenge your desk knowledge. It is truly a unique experience. Also some UN agencies do extremely inspiring work saving lives every day and make me proud to be part of the organisation.
Cons
There is no clear career progression. As part of the Young Professional Programme you move from your first post to a second post in a different geographic location after 2 - 2.5 years. The new post is at the same career level and it is then up to you to apply to higher posts in the organisation. However there are too few such posts, especially in coveted locations such as Geneva or NY. Staff who has served in difficult locations is not rewarded for their efforts or worse is penalised because they lack the network of those who started in Geneva or NY. There is also a lot of corruption in hiring, some managers hire friends or people they know. Meritocracy is low in the system at all levels, with some exceptions. Internal processes are extremely bureaucratic and wasteful of staff's time. HR is messy and often incompetent, but mostly it is not their fault because the system is governed by a complex system of byzantine rules. Some people get lazy and never leave Geneva or New York, others bravely go to serve in difficult locations but then get stuck there with very little support from the organisation. There needs to be more fairness in the way staff moves around locations. I think people who are only interested in headquarter jobs should not consider a career a the UN.