Pros
Work-life balance is great. The work is not demanding (though, see below). The people in my department are very pleasant to work with, for the most part. In fact, I might venture to say that MITRE is a bit of a gilded cage -- talented people will not be challenged, but neither will they be subject to the vicissitudes of a roiling job market. You will not have to work long hours for fear of your job.
Cons
There is little genuine "technical" work to go around beyond the usual (and uninteresting) IT and database problems. Much lip service is paid to the provision of something called "technical excellence", but in truth this is just an empty management mantra. The bulk of the available technical work is performed by low-ranking (read: < AC4) engineers, many of whom have advanced degrees, some in the hard sciences like physics or math. Of course, nothing these people say or do is listened to or respected by senior management, most of whom do not possess these degrees, or else acquired them under conspicuously non-rigorous circumstances. Further, there is little opportunity for PhD-level scientists and engineers to advance. They are paid significantly less than their non-technical peers despite doing most of the difficult work, and they are accorded pitiful raises even when their reviews are exemplary. This inequity becomes apparent very quickly to new technical hires, many of whom turn right around and leave MITRE for greener pastures within a year or two. This is tragic, really, because MITRE is losing talented people who could do much to enhance its reputation and advance its mission. Finally, the middle and upper levels of management are bloated and mostly incompetent. I appreciate that task leadership can be difficult, but the majority of our managers don't even try to do their jobs. There's a reason MITRE has been targeted by its counterparts in the private sector for transgressing the bounds of appropriate FFRDC work -- our task leaders and upper managers are incapable of handling sponsor relationships. They lack the discipline to say NO to work that falls outside the terms of our charter; they lack the courage to tell our sponsors the naked truth about their problems, instead allowing politics to dictate the message. There is therefore little to distinguish us from our non-FFRDC peers, who rightly view us as competitors.