Pros
Very good benefits. If you are a graybeard, MITRE has an unusually large percentage of people who are subject matter experts with 25+ years of experience, so no age discrimination. They also have a lot of the more common less experienced younger people. MITRE is always very highly rated by Fortune magazine and other ranking sources, including this web site.
Cons
MITRE is a "Principal Engineer" company -- meaning that all the real work is done by people from entry level up to Principal Engineer. Once you reach Sr. Principal Engineer, you are expected to do a lot of management stuff, which is OK if you are into that sort of work. MITRE is the world's most process-oriented company -- meaning that they do a huge amount of work developing processes of all kinds for the government. I suppose someone has to do it, but don't confuse that with real technical work. If you want to go somewhere and be invisible and stay until you retire or die, that's easy at MITRE.