Avantages
Lockheed Martin is BIG, with offices in many states and countries, and many different office cultures. You'll find a suit-and-tie culture around Washington DC, jeans in the western and mountains sates, khakis and button-down shirts throughout. Production facilities tend to be very deliberate and detail-oriented; as you need to be when your products are those of a major aerospace company. There are also research labs which are more flexible and tolerant of failure, but are more selective when hiring. Bring your A-game if you apply. I can honestly say LM takes ethics and diversity very seriously, making for a comfortable work environment where you can feel respected and empowered to do the right thing.
Inconvénients
The cons are about the same as what you would find working in any large organization. Depending on your business area, you may sometimes find some top-down/command & control leadership styles that can feel overbearing. Decisions that get made for the corporation as a whole can sometimes feel like they don't quite fit with your local working group. That said, these issues are usually temporary in nature, and seems to grow more rare later in my career.