Pros
Decent offices, people are nice, and if you like to cruise at work and avoid performance, achievement, or relevance (because that means work), this place is for YOU. MITRE is a terrific place for has-beens, used-to-be-someones, and those who like to say "R&D" but will rarely if ever do any actual R&D, and those allergic to delivery. The best pro is it all "sounds" good without actually BEING good, so its a win-win, right? It's a cheap and easy play to get to nice benefits (retirement bennies are nice - 403b and 401a are sharp, and if you can ever explain how they work outside of maxxing them out to just move past the paperwork and odd math, they'll make you the CEO - I think that's how the current dude got the job). Healthcare used to be good - that now sucks. The education bennies are the best in town, if you can survive the period of servitude that come with it (see the cons on what happens to your skills). MITRE isn't really a business - it's more like a quasi-government stopping point for people who are looking for that old retired-defense-employee feel (I can go to a job site and blather about how it used to be) and yet claim they work for a company. Really, its none of these things...it's the worst of all of them.
Cons
Your future, your value in the workplace, and your ability to achieve and deliver real value will suffer. I met someone in their first year who told me, "I can FEEL my skills weakening when I come here, and I worry that will continue if I stay." I thought she was being dramatic, until by the end of my 1.5 year mark, I felt it as well. By year two, I couldn't shake it, and I tried to see how I could work through it. That lady quit soon after our chat. I made some progress, but to be sure, giant MITRE mediocrity crept in and swept away my progress. There are many, many managers, but scant few leaders. What MITRE used to pride itself on - being a slush-pool for smart people (that also didn't believe in up or out) that the gov't could use is a Cold War long dead dream. The current crop of government leadership have no idea what an FFRDC is or why they should care, why it matters, what it should do, and amazingly, MITRE "leadership" also shares this same problem in itself - they have TOP people still thinking about it...zzzzz...zzzzzz. MITRE wants to be commercial and win work outside gov't, and maybe this is really a "pro" since it will merely hasten its demise, but I'm getting ahead of myself. THAT PART IS FOR ADVICE TO MANAGEMENT.