Prós
If you are able to find a good group within the company, junior engineers can get tons of diverse experience and responsibility. If you want a job that can help you build a resume for your next job, LM is perfect. Probably the best part of working for Lockheed is that there are thousands of groups within the company and you are reasonably free to move around until you find one that you like.
Contras
When you start at Lockheed, you'll hear people joke that we're the company that Dilbert is based on. After a year or two, you'll find out that those people weren't joking. While Lockheed can shower responsibilities and experience on young employees, all promotions are based on years of experience and all raises are based on sucking up to management. This is the sort of company where you could single-handedly win a $25 million contract and be turned down for a promotion because you are 1 month of experience short of the requirement. By the way, that's not hypothetical. In my time at Lockheed, I've seen situations just like that play out... twice! Lockheed is a silo'd company, so it really feels like a loose alliance of hundreds of small-medium companies. It's sort of an interesting place to work, until you realize that all parts of Lockheed fiercely compete with each other. We operate like a pack of wild animals, constantly fighting each other for food, but remaining together for survival. This creates a toxic, ultra-competitive environment where people are rewarded for betraying the guy down the hall because he's from a different part of the company. I've seen people get their yearly raise reduced for helping other parts of the company win new business. The last important thing to know is that they've been making small benefit and quality-of-life cuts every few months for the last ~3 years. They're usually small, but they keep happening like clockwork and they're adding up. I've had multiple raises and a promotion and my total compensation is the same as when I started. In essence, good performance is rewarded with just enough of a raise to keep your salary from shrinking.