Ventajas
Being a large company there is a great deal of job security. If you are interested in a non-technical career they will help you get there. If you like a corporate atmosphere this is a good environment.
Desventajas
The only way to advance your career is to go into some sort of management (and so everyone is pushed into that against their will). Promotions and compensation are based more on how long you have been in the workplace than your ability. Experience at other companies is valued above experience at Lockheed Martin. The benefits are becoming more and more a burden on the employees. Job duties do not tie into title or salary in any way whatsoever. Bottom line and stockholders are more important than individual employees (although this shouldn't be surprising in a large corporation).