Pros
Chance to see the internal process of international relations at work if you end up in peacekeeping or political affairs. Good pay and benefits for comparable positions. So much time off you'll likely feel too guilty to use it all. Once you're in it's pretty much impossible to get fired, but I'm not so sure this should be a Pro as you have to work with people who likely should be.
Cons
Bureaucracy is intolerable. The most successful employees are those that can manage the endless workarounds. Old-fashioned distinction between admin and substantive ranks. Difficult career advancement makes it nearly impossible to rise from the lower administrative ranks up to the professional without leaving and returning above the ceiling. The UN is a good place to end a career, not start one.