Ventajas
Absolutely a sense of mission with some really motivated people. Some very cool jobs available. You feel your job is very important, and that you're really doing something that matters. Absolutely no chance of getting fired unless you do something horrible like racially or sexually harass someone. If you want a 30-year employers with many different types of jobs, CIA is great.
Desventajas
Many levels of management are almost uniformly awful. All levels of employees are constantly sent to BS training courses. Managers seem to spend half their time in management training courses which are clearly counter productive. Bureaucracy as expected of any federal government agency. The absolute worst thing for me was that the best and the worst employees were treated exactly the same in many cases. Same salary, same job titles, same seniority. Need a way to weed out horrible employees.