Ventajas
I worked for Lockheed for over 30 years and retired last year. Since Lockheed is primarily a government contractor, they are not sensitive to market swings like most other companies. I was only laid off once twice. Once in the early 80's and again for 6 weeks in the 90's. I started as an hourly employee then salary. There are many job opportunities there. If you are willing to get educated and are open to moving, you can travel the world working for them. They have employees representing every product they sell to the military. So, wherever the military is, so goes Lockheed Martin.
Desventajas
Like most large companies, it's easy to get "lost" to management. So, one must get educated or earn certifications (IT). Also, they don't really want you working forever. It's best to retire ASAP. Who wants to work in their 60's anyway. Also, they should drop the requirements for college degrees. Most IT repair or admin jobs don't require a degree. But, you need to get as many certifications as you can.