Pros
Working at JHU/APL allows for the opportunity to work on programs of national importance while in an employee-first environment. Benefits are excellent and include tuition reimbursement, with a large amount of flexibility in choosing graduate work. Flexible working hours and generous vacation policies are additional benefits. Co-workers, while not the most social, are very accomplished technically, and there is a lot to learn from them all. New staff have opportunities to get face time with sponsors, present at conferences, etc.
Cons
Many of JHU/APL's projects seem to be directed towards solving problems in national security that may have had more application during the cold war than today. Many managers have technical training only, and are not as effective at managing people. There is also a divide between younger technical staff and older staff with military backgrounds and it can be hard to jump into management without other industry or military experience. Sometimes the flexible work environment actually is so flexible that it's hard to get team members to take their assignments seriously.