Pros
The UN has a very exiting mission and offers a very large diversity of jobs and functions, and many very exiting tasks related to assisting people in need all over the world. For good people with excellent skills, who know how to network and who can learn how the system works, this organization provides excellent career opportunities. However, taking initiative is key. The benefits and renumerations are still quite good for many places in the world with very few exceptions (like Switzerland, where local salaries are still way higher).
Cons
Remember, this is a bureaucracy that has grown in over 65 years. It is important to understand how the system works, and the interaction with stakeholders. This can be difficult for some. There is still lack of transparency in many processes and there are places where "the own people" are promoted. There is a repeated drive from Senior management for increased "mobility" of UN staff. Various models have been tried with mixed success. (e.g. the "forced mobility" started in 2002 and first effective in 2007 seemed to have been failed; the voluntary VINEs program was considered a success by many staff members, but Senior Management thought otherwise and seems to return to a more forced approach). There is a clear trend towards an erosion of staff rights and benefits, including imposing new and difficult conditions for promotions, abandoning the permanent appointments, etc. While I would consider conditions still very good, the trend is evident.