Pros
MITRE's public-interest mission is unique and makes one feel proud to work for the company. One feels that one is playing an important (sometimes critical) role in improving US Government programs and activities. MITRE work offers opportunities to have impact that one simply cannot get anywhere else. The company is very family-friendly, and it's not hard to have a satisfying career that allows one to maintain a work/life balance. The company is a not-for-profit, so one isn't constantly fighting to get the next version of the product out the door. The retirement benefits are excellent--a company matching program that exceeds 12% of salary. One is rarely stuck in an unsatisfying job--projects typically change at least one a year, so if your current assignment doesn't make you happy, just work with your manager to look for something different. You might not get something you like right away, but new opportunities do arise on a regular basis. The information technology support is top notch, excellent internal networking infrastructure, excellent support services. Primary work computers are regularly turned over (2-3 yr lifecycle). The facilities, particularly in the Washington DC area, are attractive and well maintained. The culture tends to favors those who are entrepreneurial--in many ways one can make one's own career at MITRE. The company is in many ways a small marketplace of skills, so if you have and continue to develop your skills, and invest time regularly in finding and connecting with your "market", you can have years of steady, diverse work.
Cons
Promotion opportunities in purely technical roles end at about the Applied Capability 4 level, after which time one must become a manager in order to advance further. Managers typically have no budget or strong incentive to really manage, as all the engineering staff is assessed based on technical performance. You get what you pay for. The mid-level managers do their management "on the side" so they can focus on technical deliverables. Work opportunities in the Bedford area are becoming harder to find, as the vast majority of MITRE sponsors are located in the DC area. Most of the "plumb jobs" are in DC, as they typically require on-site working relations with US Government officials. The company used to have a strong internal research program, but it has recently transformed into a collection of short term projects with little appetite for sustained focus on strategic problems.