Pros
Work life balance is very good, but if you are passionate and working on difficult problems, be ready to occasionally pull all nighters or work weekends. Work from home--depending on management, trust level, and what your job entails, you can have full autonomy in deciding where (and sometimes when) you work. If you are a junior employee, you are almost guaranteed a promotion within 1-3 years of employment (assuming even average performance). MITRE believes in the potential of its employees and invests accordingly--many interviews are behavioral and toxic culture of whiteboard coding is non-existent. It is very difficult to get fired as long as you account for your work hours (I have only ever seen people "booted" from teams but never fired). There is an opportunity to grow technically in almost any direction due the diversity and demand of MITRE's work, and opportunity to learn new skills while on the job.
Cons
Salary increase outside of promotions is non-existent at an average 3% raise per year--if you want fair pay after your 1st year, you will need to get promoted often or simply find a new job. There is incredible variation in the quality of MITRE employees due to the lax hiring process. Promotions are difficult to obtain and it is common for some employees to go decades between promotions--to get promoted you generally need to play politics (i.e. be friends with influential management) or being an incredible research leader (i.e. capable of expanding MITRE's business / research programs)--politics is often involved in the latter when deciding who gets money to lead their own projects. There is a high risk of becoming obsolete to future employers if you are not proactive in using the latest technologies for your projects--many managers and project leaders simply manage and are out of touch with how a project is actually executed by staff. There are no standards for coding--if you want to be employable outside of MITRE, you will need to discipline yourself and write maintainable, documented code. MITRE has a lot of fat to trim: there are many senior staff who perform just enough not to be fired but don't actually provide any value and they are too old to get jobs outside of MITRE. If you are a star performer, you will find it very difficult to leave MITRE, as management will do anything they can to keep you if you're the glue holding multiple projects together, and you'd probably end up in a more junior role at a different company if you left. MITRE has an antiquated system of coverage--you need to account for every hour you worked to each project. Less desired employees are often always scrambling for full coverage, which creates a toxic culture that encourages employees to work on projects they otherwise wouldn't, simply so they don't get fired. A lot of upper management is both incredibly technically incompetent and paid incredibly high, relying on over performers to complete their work while the management takes all the credit--MITRE could probably lean up and save so much money all remaining employee's salaries could easily be doubled. MITRE's perks are below average compared to other tech companies--all meals should be provided for free.