Pros
LM is huge. You can find your niche here, whatever it may be (production vs. R&D, technical vs. leadership, etc.), though it may require some active searching on your part. Salaries on average are better at large companies vs. small ones. There is a strong sense of pride in what we do. Building systems and capabilities to find bad guys and protect our fellow countrymen arguably provides a greater sense of purpose than, say, developing the latest iPhone app. Career advancement opportunities are there, provided you find a good manager to support you. The corporate culture being flowed down from the top is grounded in integrity and doing the right thing. Having had the opportunity to see upper management in person, I believe it is genuine. There is increased oversight and restrictions on who gets into management and who stays. In recent years, I have seen two managers removed from leadership. Neither of them looked out for their direct reports well and one of them displayed questionable ethics. I consider it a very good thing that the leadership bar is being raised.
Kontras
Nothing LM-specific, just typical large corporation issues (I have worked at several). It can be a challenge finding the right area and group for your interests because there are so many to choose from. On large programs especially, it is easy for dead weight to hang around unnoticed (perhaps some people would consider this a Pro), and it isn't easy getting rid of them. While average salaries may be higher at large companies, it is probably easier for top technical talent to move up quicker in smaller companies.