Pros
Relatively immune from recession; good tuition reimbursement program for grad school, location, fairly decent health care; low stress, no working nights/weekends/Friday afternoon (usually); lax dress code (if you're in engineering, at least)
Cons
Because of how Lockheed Martin grew (acquisition), working at some of the business units just doesn't feel like working at a Fortune 500 company. It feels like you work at...whatever was there before it was acquired. Additionally, chances are that you will be completely isolated from every other business unit and site. Transferring from one business unit to another, or one site to another, is NOT as easy as it sounds -- one pretty much has to threaten to leave the company to do so. The worst feeling is the feeling that technology is just passing you by - we use software until it goes end of life. It's just not a "cool" place to work, but you already knew that. Incompetent employees are shifted around or even promoted. Lots of talk about horizontal integration, but virtually no action.
