Pros
MITRE is a great place to get a foot in the door since they'll hire just about anybody to keep up with attrition in the middle engineering ranks. MITRE is a great place to finish your career due to generous retirement benefits and the fact that they'll hire just about anybody with industry connections.
Cons
There is almost no chance of getting a meaningful promotion if you start off at a low rank (AC2 or 3). Since becoming an AC4 almost definitely requires you to stop doing technical work and start managing projects, engineers are pretty much capped at being AC3's (with a few exceptions). This means that salaries severely stagnate compared to anywhere else in the industry. Add to this the fact that MITRE's middle and upper management is severely bloated by people who would most likely be unemployable anywhere else, and the chances of moving up are almost nil. If there is an opening, there is a 90% chance they will hire somebody from the outside than promote from within.