Ventajas
Great toys, motivating mission. A very talented and dedicated workforce. The standard benefits of working for the federal government if that's important to you -- health care, pension, and you can't be fired.
Desventajas
NASA, despite it's reputation amongst those who have never worked with it, is primarily a government bureaucracy. It is not the organization that gave us Apollo. The technology used in mainline spacecraft development programs is many years behind the current state-of-the-art, due to an overly risk-averse, conservative, and bureaucratic approach to engineering & program management. Perhaps due to advanced age of most of the workers, there is tremendous resistance to any solution that isn't similar to what has been done in previous projects. Due to civil service protections and lack of new programs, there are few management or leadership opportunities for young engineers or new hires -- you must wait for many years before a position above you clears out before you can get a position that can "make a difference".