Pros
Before the financial crisis, this was a great place to work at, great compensation, great benefits, good work-life balance. The housing downturn hit hard and - while being an interesting place to be in, in the middle of the fight - the stress and uncertainty levels shot through the roof. Those who can weather the storm could very well be rewarded in the end. Fannie Mae might go away in its current form, but cannot disappear. Interesting place to work from a professional point of view, if you care about credit and valuation models. The benefits are great and compensation is decent. For the Washington, DC area the alternatives are slim.
Cons
Uncertainty about the future, layoffs after layoffs, politics. Very fat middle-management layer. Everyone pushes to be a director, since in a non-management role you can hit the ceiling fairly quickly. Having a deeper non-management track would go a long way towards solving this and hanging on to the quality people who actually do the work, not just talk about it.