Pros
"Employees first" is one of the company's core values, and they practice it. The company treats its employees well, with good benefits, an ESOP, and lots of freedom. Even when times are tough (like during the budget sequestration and government shutdown scares of the past few years), much effort is made to keep everyone on board and gainfully employed. Customer problems are interesting and satisfying to work. New contracts are pursued mindfully and deliberately, with a focus only on those opportunities where the leadership feels that the company can bring real value to the table.
Cons
There are many senior people with loads of experience in mid-level and senior positions who probably aren't going anywhere. Opportunities for advancement are limited.