Pros
You get the chance to learn about one of the "hottest" products in finance - MBS. The culture of FNM is very government-like wherein many people work 9-5 and there is ususally early release the day before holidays. As long as you finish your assigned work you will be free to determine a schedule that works best for you (unless you are in an accounting role).
Cons
Fannie is extremely siloed in that different groups do not communicate efficiently/freely, resulting in a good deal of duplicity in analytics and effort. Because of the continued public scrutiny and ongoing debate surrounding FNM's future structure there is always a sense that things are being "paired down" and directors need to justify their groups resources/staffing by pointing to the quantity (rather than quality) of work that a group does. The government-like culture also comes with a great deal of bureaucracy.