Pros
Incredibly interesting technology to work on. There's more variety here than most places for the technically-inclined... Circuit design, software, hardware/mechanical, systems, etc. They do it all.
Cons
From an execution point of view, Mercury Systems leverages old, antiquated engineering practices and is extremely poor at planning, resource management, maintaining a long term technical vision and executing on it. They do great things but could be so much better at it. The company is positively horrible at resource management, having adequate resources to maintain continuity on a program, focus, allowing time to sufficiently and accurately plan, etc. Some of these problems are due to the nature of the work but it could be much better. Change at Mercury is glacial.