Pros
MITRE treats its employees very well in terms of benefits. There is great career development, and typically a nice working environment. Salary is generally good and comparable to the market, and work-life balance is the best around.
Cons
In my experience, MITRE struggled to provide quality work. A few quality projects (which are actually useful) are financing an incredible amount of complete nonsense across the company. It has 20-30 times the number of employees it needs; every project has too many cooks in the kitchen. The culture is one of faking it; very little cutting edge or interesting work is done, yet all projects are somehow framed as successes. It's internal research program is a complete disaster and a sad use of tax-payer money. Models and prototypes are often built for absolutely no reason. MITRE occupies an important space in between the government and contractors, but this former employee is giving it an efficiency rating of 10%. And with your taxpayer money, you should be concerned.