Prós
It is fulfilling to contribute to the defense of the United States. You may have access to technologies and projects that just don't exist in the private sector. Usually there is a good emphasis on work life balance. It is a huge company that has a lot of interesting projects. Lockheed makes missiles, satellites, ships, and the best military jets in the world. The benefits are above average. In general the people you work with are very patriotic and nice. There is a sense of pride in working for Lockheed Martin and you generally won't be ashamed you work for them.
Contras
You will see firsthand how much waste, inefficiency, and fanboy jingoism exists in the defense industry In the non-defense sector Lockheed would be eaten for lunch. Too bloated, too full of itself.