Pros
Work can have significant impact on important government decisions (level of impact varies by project). Great work/life balance. I routinely take all my PTOB when I want. The flexible work environment is fantastic. Amazing retirement contribution. I have personally had great career success (in terms of projects, promotions, and salary), but your mileage may vary. MITRE rewards self-starters who are willing to take leadership roles, develop good work, and have high technical skills.
Cons
Projects vary immensely in terms of goals, technical difficulty, and level of impact. It is impossible for everyone to work a high impact project at the same time. Many projects do not use the latest and greatest in technology. It is possible to fall behind the current state of the art. Then again, MITRE's primary sponsor is the USG who is often not using the newest technology. Often times, MITRE's role is to help modernize the USG.