Vantaggi
Lockheed Martin is a great place for recent graduates to start their careers and prove themselves. From that respect, they are actually pretty good at providing opportunities for people where other companies might want to see more experience. Their sheer size also provides a lot of breadth as far as opportunities to work with different government agencies/customers, various technologies and geographic locations.
Svantaggi
Not so great if you're looking to grow your career (especially with regard to salary growth). Lockheed Martin is known in the defense contractor space for under-compensating employees. You would think this means cost savings for the government, but in reality most contractors charge about the same, meaning Lockheed is just skimming more off the top for themselves (and to pay upper and middle management cushy salaries.) You're better off finding a smaller company, with smaller overhead that can afford to pay you a better salary. Also, it's a crapshoot when it comes to the "work/life balance" they advertise. I worked on a program where you were expected to put in 60-70 hour weeks (5 hours of which you worked for free), but I also worked an IRAD which was a typical 40 hour week. Corporate culture is standard big company culture. The shareholder is number 1. They will do everything they can to maximize their revenue (how much they charge for you to the government) and minimize their cost (how much you get in salary, how many days off, fringe benefits, etc.) The system is set up to reward the top management more money and stocks to treat their regular employees poorly and convince them to stay for as long as possible.