Pros
The focus at MITRE in my experience has been that we are focused on results. Yes, billing time matters, but the culture is very flexible on when and where you do your work. Just get the work done on time. We are very focused on impact to the customer, and look for ways to make sure that our work stays that way. we stay away from work that is not "MITRE work", and do our best to work with our sponsors to understand what that means. The infrastructure around the employee at MITRE is solid. I don't have to worry about any of the administrative problems that I've experienced at other companies in terms of getting employment and benefits paperwork in order. Very organized workplace.
Cons
Sometimes working for the federal government as the client, things can move very slowly, and project requirements are often very "squishy". If you are the type of person who needs things to be clear cut and laid out for you to be effective, this may not be the best environment for you. However, if you are good at making sense of a confusing space, and like to set direction, this is perfect. If you're looking for techincal development, make sure you get into the right department at MITRE. Many departments are not heavy duty technical, as we do not do implementation. But there are technical departments where you can exercise that skill. Just make sure you get into the right place if that is what you want to do. and even then, it's more research focused, not day-to-day coding and implementation.