Pros
MITRE is a large company that performs a large number of different technical jobs for the government. There is a large amount of flexibility to change jobs within the company to find a better fit for your particular requirements. If you enjoy technical work, such as engineering (systems, software, etc), software development, analysis and the like, there's probably a job a MITRE that you'd find enjoyable. There are great benefits, including the retirement plan and ample paid time off. There is quite a bit of stability in the company as an FFRDC as compared to typical government contractors. There is also a good work/life balance promoted and supported by the company.
Cons
Career advancement seems to be on the slow side when compared to many defense contractors. Pay is reasonable, but also not as high as comparable contractors - particular as compared to other contractors doing classified work.