Pros
Huge company, lots of transfer opportunities if you are in a job that isn't a good fit. Fairly good job security, but layoffs are on the horizon. Good name to have on your resume. If you're coming out of college this could be a good place to work for a year or two, at least until you get a security clearance and can get a better job elsewhere. You don't actually have to be any good at your job if you're good at sucking up to management. I've seen people go months without accomplishing anything of value and not face any consequences. If you're looking for somewhere to coast, Lockheed Martin is paradise.
Cons
Huge "good ole boys" network. Promotions and raises are based entirely on who your friends are within management, and not on how good of a job you do. "Performance" assessments are one "rating" per year based on vague goals like "energize the team" and "shape the future" that are intentionally wide open to interpretation, giving management cover to justify giving any employee any performance rating. Employees who question being asked to do significantly more than other employees without recognition are threatened with termination if they don't put up and shut up. Management insists that Lockheed only hires the best and brightest. That may have been the case at some time in the past, but today many employees are incapable of producing high quality output, and only good at deflecting blame away from themselves when "it hits the fan." These people are usually promoted into management when it becomes impossible to ignore their technical incompetence.