Ventajas
Lockheed Martin is a great place to start a career - one of the benefits of being a huge corporation with varied departments is that there is a lot of potential for advancement. I imagine that job security is a bit better than in other industries that hire for similar positions because defense contracting is a fairly steady business - plus, security clearances are fairly expensive to attain, which means that from day one the corporation has invested quite a bit in even its entry-level employees. This also helps in the job market when looking forward. The success of any project and thus the success of the company relies entirely on the ability of its employees, so LM spends quite a bit to keep morale levels up and to make sure that its employees are well trained. The corporate culture is fairly laid-back and casual, but not too much so. Everyone is on a first-name basis. Dress codes and schedules vary by location but are usually pretty flexible. Everything is task oriented - as long as the work gets done, management doesn't nitpick about minutiae.
Desventajas
I can't really speak in detail about what I do when talking to friends and associates from outside of work. Also, salaries are on average slightly lower than in the private sector, but by no means unfair. Sometimes, the strict requirements of government work call for overly tedious work processes.