Its either really good or really bad.
Pros
There is a lot going on and if you are there to grind and get work done you will be rewarded accordingly. If you are on the business side of Fannie Mae you are going to go far in your career even after Fannie Mae ceases to exist. You will get first hand experience and insider knowledge of how the future of the mortgage industry will be structured and function.
Cons
All the good things depend on what group you are under and what manager you have. If you are on the IT side of Fannie Mae you are one day not going to have the job you have now. IT is completely being outsourced to companies like IBM and other entities. There are some hard to deal with management there that will not change their ways and they will get rid of you if you get on their bad side. A lot of corporate politics. Its all about whose good side you are on and if your are marketing your self well within the company. You cant just do good work and think everyone will notice.