Ventajas
flexible work hours & schedules many varied opportunities to work on different types of projects cutting edge technical efforts, won't find anywhere else in the country/world experienced knowledgeable professionals to work with & learn from respects vacation time, allows you to take it when you want to
Desventajas
Company focus is as a large, complex system integrator. Does not make the majority of technical components themselves, instead manages variety of subcontracts to "integrate" and deliver a system. Therefore, the real design and engineering work is often done by another company. If you do not actively seek out new & challenging positions within the company, after a number of years you may find that you've lost your core technical, engineering & design skill sets in favor of more broad systems engineering skills and general experience in providing oversight to subcontractors. In short, one complaint I've heard is that some 30+ year veterans of Lockheed are useful within the company but completely useless at a different job where you actually have to design things yourself. Also, the work-life balance varies between departments and programs. Overall, it is positive, but in some areas (especially Special Programs) it can be very negative for the 10-30% of employees who have positions with a decent amount of responsibility. It seems the work-life balance is consistently absent for the top-level experienced engineers. Last, in 5-10 years there is a significant age gap that plagues all of aerospace. When those people retire, a lot of knowledge will go with them. Things can be re-invented and redone in different and better ways, but I doubt that transition will be an entirely smooth one.