Pros
Pockets of interesting work that also exists elsewhere in the market. A 'trusted' relationship as a 'non-for-profit' with government sponsors. Access to many government organizations that individual companies don't likely have.
Cons
First, the overwhelming negative reviews are correct. No need to reproduce them here in detail; only a quick acknowledgment: (Paraphrased: CEO sucks, attrition is off the charts, disdain for senior/knowledgeable/technical employees, no training budget, reduced benefits, and a race to the bottom.) Second, the occasional 5.0 positive reviews that are interlaced here-and-there are clearly company HR representatives. There is no way an employee can be both conscience and honest to rate the CEO or culture at MITRE as 'good'. The CEO rating is a weighed average and he's dropped 20 percent over the last year. For you math folks, that's a LOT of recent zero ratings right there. Rather, this review is meant to serve as a public service announcement to those seeking employment at MITRE. Don't do it. I haven't met you, I don't know you, but I know you deserve better. RUN. Currently, 1 out of 3 MITRE employees have been with the company for more than 3 years. Those of us 'old MITRE' are actively looking (and finding) other opportunities outside of MITRE. This is going to make it harder for those of you that are coming newly to MITRE. The MITRE back-plane (management, project leaders, group leaders, department managers, etc.) are evaporating. The people that are now filling those roles are inexperienced. They mean well, but they don't have the wisdom yet to know what they've signed up for. They will be burning themselves out; guaranteed. This churn will not serve you the new employee well.