Pros
This company has done and continues to do cutting edge research an development across a broad spectrum of technologies. If you want to see tomorrow's technologies today, work at BBN (or a company like it).
Cons
The business model of a small S&T company (like BBN) depends on selling ideas and research to the government at a competitive rate. Since the output of this work is often only a research report rather than a deliverable working product, the research has to be very promising and price very competitive (similar to academia). This is distinct from the business model of a large defense contractor that often enjoys a near monopoly on its goods and services (like Raytheon). This fundamental tension has been the source of considerable angst within BBN since its acquisition by Raytheon and it remains to be resolved.