Ventajas
The biggest reason is the type of work and customers you get to work for. I feel a sense of pride in producing a product that will be used to help the country, state, local town, and the people I care about. They do a pretty good job hiring good and competent people. I really enjoy the people I work with. Obviously, no company is perfect, but they have a generally good knack for weeding out the bad ones. Also, I think they're run much better than their competitors. From dealing with subcontracts and competitions and the such, I'd much rather work for LM than for Raytheon, Boeing, or Northrop. Very open to the work-life balance. Good to great benefits in my opinion. The engineering environment is very healthy. Teams generally work well together and it's preached from the top down.
Desventajas
Not all of the first level managers get it, or understand job performance. As with any big company, you have to deal with some bureaucratic bullsh!t, but it's expected, and most of the experienced engineers do a good job teaching you how to work with it and get your work done. Generally salaries are probably lower than other companies, but the benefits I think make up for it. Corporate buzzwords (again, this will happen with any big company)