Pros
I've been with LM for some years now. It's always been a great place to work - fascinating problems to work on, great sense of mission centered on national security, good pay, good benefits, and plenty of autonomy in my work. The company is getter better too, and has been at an accelerating pace over the past few years. We now work just 4 days a week for 10 hours a day, telecommuting is strongly encouraged, there's generous PTO, and the company is quickly modernizing its workflows. In a year, I might be looking to give the company 6 stars!
Cons
The only con I can think of is a certain amount of inconsistency across the enterprise in terms of company culture. LM is a big company, and in some respects, a collection of companies. Regardless, it seems folks' experience can be heavily influenced by their team - do they have a good leader or bad, does the team work together or compete, etc. The company tries to encourage consistent good behaviors, but there is still some variation, and not all teams are as positive as mine have been.