Pros
There are still really smart and capable people around. The culture is very friendly and relaxed, but you are still able to intellectually challenge yourself if you want. There is also a new executive leadership team which has been making reasonable choices so far for the hand they've been dealt. The work can be meaningful if you maneuver your way to good projects, and they have a good internal R&D program for research-oriented employees.
Cons
The company under the old executive leadership team made bad choices, leading to its current state of deteriorating culture and financial health. Currently the company is a like a very rocky ship, and no one really knows if MITRE can weather the storm. Even before the storm, MITRE struggled with keeping up with state-of-the-art technology and business practices. The experience varies greatly with the team and department you're assigned. There is a massive amount of heterogeneity in the company, making it difficult to really state how good or bad or valuable or worthless your work can be.