Avantages
Great benefits packages in terms of vacation time accrual (it never expires, but does cap at 400 hours). Flexibility is awesome. I work at a site with off-Fridays every other Friday. There are some sites that work 4x10, so they never work Fridays.
Inconvénients
Very large company with a lot of politics. Very slow moving. Hard to introduce change. And once change is introduced, if you move on to another project/job, expect the changes you implemented to be undone. Resistance to change is rampant throughout the company and it will take a while for them to be able to respond to customers quickly. This works for them as a defense contractor. In the commercial world, this would result in failure. People in leadership position often have no leadership skills and made it to their position based on seniority and technical experience. Being a good engineer does not equate to being a good leader of people.