Pros
MITRE provides very good benefits, a reasonable salary (not location specific), allows telework, and respects the work-life balance we all need. You do get to work on some pretty cool tasks ... if you're lucky. Work environment, support staff and computing equipment is all good to very good which can make your work life much easier. High percentage of staff with advanced degrees.
Cons
The performance review and salary administration is a mess that many try to avoid even dealing with. Management consists of previously good technical people that refuse to be managers and want to keep being technical staff. Management is scared to deliver the bad news or make the tough decisions. If they ignore it, hopefully it will go away. If you aren't part of the in crowd, good luck getting on the good projects or getting anything like independent R&D type activities. But once you've got it, you'll have it year after year. Dead weight stays around for far too long.