Ventajas
The best thing about working at LM is the products that are made. It is really awesome working on spacecraft. Because of the unique dynamics that come up with the space environment the projects are always different and changing so it prevents you from getting bored. Its also a large company so you can try different things. They also have a very good flextime option, you can do a 9/80 or 4/10s and flex hours between days in the given week.
Desventajas
The pro of different and changing products also means that you have to constantly find new positions. Once a product is launched, the program is over and you need to find something else to so. Sounds good and only benefits people with a good network. On that same line, it doesn't matter how good you are at your job or what is on your year end review, if someone doesn't like you then you are screwed. They also have been having a low retention rate of engineers (thats my field so I can only speak to it), a LOT of the ones that hire in right out of college leave between the 5-10 year mark when you get stuck in the ranks and have little hope of a promotion, that's where I am currently. They are starting to push the older ones out by changing our benefits, which also hurts the younger ones. Never trust a benefit that they sold to you to make the job seem desirable, its probably not as good as they claim and will be changed the next year anyways. In my 10 years, I've never seen a change in a positive direction for employees. Since it is also a large company things take a long time for new ideas to be implemented so be ready to step back in time.