Pros
Great family life balance can be had at MITRE. Employees can adjust their hours around their children's schedules. I don't miss a thing with my kids lives and I wouldn't trade that for anything. In addition, the work is very intellectually challanging and employees are given the opportunity to research ideas that are outside of conventional thinking. If you can network well (and this is the key to success on many levels), you can have much control over the projects you work on and the career path that you want to take (technical, managment, etc) There are lots of smart people from many different fields that you can learn from. And most people are easy to get along with and generous with their time since the FFRDC workstyle is of a slower pace. You can grow your knowledge across many different technical fields while working at MITRE. Main MITRE campus are spacious and have very nice facilities. MITRE is an especially great fit if you have an academic background / PhD or if you have previous military experience.
Cons
If you are not a good networker you will struggle with the promotion process and the technical quality of work from your projects. Promotions are hard enough to come by regardless of whether you are a good networker or not. All promotions take time except for a talented / lucky few, but with a 'flat' company like MITRE, this is somewhat understood. Competition for internal IR&D is overly bureaucratic and quite baffling. It seems to get worse every year and upper management seems to value the creation of a research portfolio containing all of the hot buzz words over giving MITRE's talented employees intellectual freedom to pursue groundbreaking ideas. I really feel that MITRE's management is significantly holding back the quality of MITRE's research. MITRE is slightly moving away from its traditional FFRDC / think tank role. More and more projects are requiring employees to interact in a program managment type of role between the government and its prime contractor.