Pros
MITRE has the best benefits package imaginable and provides real stability over the long haul. It's possible to find great work to do with high sponsor impact, work that you'll be proud of having done. With care one can find great colleagues and mentors to work with, though the new hire process doesn't reliably produce great results for junior tech staff. (Sometimes it's hard to know what junior staff ought to do at a place like MITRE).
Cons
There's a stagnancy that many others have commented on. It is difficult to see/understand how to progress with a technical emphasis at MITRE. Folks who can deliver flash and bluster (passing it off as business/management expertise) seem to advance leaving others with less flashy but solid tech and engineering management skills behind. The internal R&D program is baffling (both in terms of investment areas and process). While you largely make your own destiny at MITRE, choosing your "management" is a large part of controlling your advancement, something that can be hard to see for a long time.