Pros
Retirement benefits are excellent! Vacation time is pretty good too, but includes sick days. Good for people who never get sick. Flexibility to occasionally work from home (snow storm, sick child, etc.).
Cons
No career growth on the Corporate side at all! I worked in the same position for 5 and a half years and never received a promotion or any kind of change in assignment despite big promises at the interview. It was not for lack of trying! Big part of the reason why there is no growth is that people stick around forever, which is a testament to the good benefits of the company. There are many people at MITRE who have worked there 20, 30, 40 or even 50 years. Some people just never want to retire, which means that very few positions open up. It also means that adopting new ways of doing things is painfully slow, because "It has always been done this way." Another con is the minimal annual raises, which average around 2% per year.