Avantages
Lockheed has very mature Dev, Test, and Production operation processes, and well-enforced operation policies, which leads to a nice, stable environment. Lockheed also supports and frequently wins new government and commercial contracts, so you'll get experience supporting a variety of different agencies & industries. Technical leadership is strong - at least in terms of familiarity with existing contract support.
Inconvénients
They're extremely cheap. No bonuses, and the below-average raises often don't cover the annual cost-of-living increase OR the increased costs employees pay for their "benefits". This is partially a result of being a government contractor, where income is somewhat fixed. Uncertainty over contract renewals can also be stressful.