Pros
Mission focus of the work and the sense that the work you do really matters and is important. The culture is pretty collegial - great people tend to migrate to Fannie Mae - those who have not only strong business skills but also have a sense of altruism.
Cons
Impossible to be promoted from Senior Manager to Director (where Director is the lowest of senior management ranks). Morale is currently very low. Recent layoffs in January are likely to be followed by more. The business is often poorly run with business decisions being made (e.g. eliminating a team) that then hae to be reversed a few months later (rehiring new staff for recently eliminated team -- I have seen this happen on several occasions. With the current layoff stress their is a lot more back-stabbing as every one struggles to prove their worth and value. Management is stressed as well and is overworking the remaining employees. There is the sense by many Fannie Mae'ers that there is no life after Fannie Mae so most put up with inefficiencies and poor management. Your experience is very much dictated by the quality of your team and manager.