Ventajas
I had great co-workers: friendly, knowledgeable, and helpful. We placed more emphasis on getting things done and correct than on development methodologies or the CMMI way. We had a short release cycle. I strongly recommend this if you can get it. I worked in an interesting field. I was given time and room to grow into my responsibilities. I got to train myself and work on a broad range of skills, and eventually lead sub-projects.
Desventajas
I worked in a locked-down environment. Tools and software could only be updated with outside approval. I wasn't able to talk about the subject matter of my work to anyone but my team. Once a year, linked to compensation, Lockheed Martin does stack-ranked reviews with a 1 to 5 scale. 1 means you vandalized your workplace, 5 means you saved starving children from the Joker, and 2 means your manager has already talked to you about your borderline performance or behavior problem. For rating good workers, that leaves 3, nothing to shout about, and 4, well done. Your self-assessment, peer assessments, and manager assessments get compressed to one bit, which is fighting with twenty other bits for attention. My workplace expected a lot of me, including mandatory overtime for one stretch. We were assured that our efforts were helping us meet schedule, then our schedule slipped by several months. I felt there was a large disconnect between my sacrifices of time and management's reward or feedback for that time. I also felt that it served no purpose. In a larger way, I felt like management let our customers push for unrealistic schedules. This put pressure on software engineering to no great purpose. It can be very corporate at times, and the BS can be a little thick at times.