A stable yet flexible environment offering fun scientific challenges (if you can navigate the odd corporate culture).
Pros
MITRE offers a great balance of work & life, and most employees have flexible schedules and reasonable hours. New employees get 23 days of paid vacation per year, which increases to 28 days after 9 years. The retirement plan is very generous, as employees contribute 2% of salary to receive a matching 7.5% contribution from MITRE. The company encourages (and pays for) continuing education. It is a stable job environment with very low turnover, and layoffs are exceedingly rare. The company's management structure is not very complex, and a typical manager's role is to assist the subordinates in finding engaging projects, rather than to direct the employee's everyday activities. Generally, employees are provided with most of the resources they need to succeed at their job. There is often a lot of freedom for an employee to choose the types of work they perform, and MITRE does fund its own internal research program. MITRE doesn't produce or sell products, so researchers are usually able to spend more time on interesting science than on mass production or support of a product. Research is project-based (usually with more than one project at a time) which can help to prevent an employee from stagnating with the same daily tasks day in & day out.
Cons
Compensation is fair, but as a nonprofit affiliated with the government, bonuses and stock options are not available. The company must also abide by many of the government's rules and regulations. As such, purchasing equipment, planning travel, and being reimbursed for expenses can be quite a hassle. The list of policies and procedures that an employee must follow in such circumstances are lengthy and filled with many potential landmines. The MITRE research portfolio has recently shifted dramatically towards immediate solutions to specific government problems, leading to less of an opportunity to perform impactful long term research. Proposing and acquiring a project funded by MITRE's internal funding agency can be quite laborious, with little visibility into the process. The upswing is that there is a lot of overhead that goes into acquiring funding, but less time to do the actual research.