Pros
I have worked at LM for 16 years now and I will have to say that they are extremely ethical and care more about their employees than their bottom line. Maybe not surprisingly, it seems to help our bottom line more than anything. Now I can only speak to my experience, but I have thoroughly enjoyed my time at LM. I've had some managers who weren't the most involved in my career, but they definitely cared about me and treating me fairly. We also get to work on the coolest missions. The salary/benefits seems to be very competitive when I talk to my family/friends/neighbors. Work life balance has been awesome for me. I've been able to flex my schedule and work from home when needed.
Cons
Classified programs mean you can't work from home very much. It's a big company, so there are chances you can get missed depending on your local organization.