Pros
Very flexible work schedule. Ability for advancement. Can learn a lot of information about a lot of different technologies by moving between projects.
Cons
Like any large company you are really just a number. Moving between projects has a formal interview process. In other words, Lockheed Martin is a huge umbrella corporation comprised of many small/medium sized companies. Can be very difficult to get another internal job. Health insurance plan is not good at all. Seems like every year more benefits erode away. With government budget cuts in defense spending... it's possible that we will see close to $400 billion dollars cut from defense over the next decade or so. This would cause Lockheed to shed somewhere in the ballpark of 12,000 jobs.