Pros
High starting salaries, especially for college/university graduates. In addition, cafeteria food was great, although the sizable lunch break (1 hour) enabled me to sample food from the area's eating option. Also, 9am-5pm (or 5:30pm when UNGA/General Assembly is in session) working hours and differential compensation (overtime) is paid for extended hours. Substantial camaraderie between staff, depending on the particular unit.
Cons
Career progression is quite difficult, don't expect to make 6 figures within 5 years of starting. In addition, seamless, etc does not deliver to the Secretariat building (although they may deliver to other buildings). Unfortunately, the UN is not the most transparent employer around and there are many hoops that have to be jumped through to get a satisfactory answer.