Pros
Most of the things I am going to write is coming from the context of being a Software Engineer/technical professional at MITRE. A lot of it can be generalized, but not all. Firstly, the work/life balance here at MITRE is very strong. Neither I nor anyone I've worked with thus far have reported needing to work late hours/weekends/holidays to get something done. Crunch is non-existent here. Secondly, the new CEO has recently made some big changes in support of the MITRE employees and their benefits that has reinvigorated many people's love for working at MITRE. This instills great hope in me that the future of MITRE will only get better. This brings me to my last, and arguably most important, point. The people that you are going to work with here are some of the kindest, most cooperative, and downright awesome people you can find anywhere. There will always be some people that you don't get along with in any place you go to, but the vast majority of people I have met so far have been agreeable, helpful, and ready to get things done. This is the place to work if your priorities for a job are serving a grander mission for the country, having opportunities to see and work on some really amazing things, not getting stuck on the same project/product for your whole career or large parts of it, and you want the opportunity to research and innovate in your field.
Cons
As with much of the public sector/public sector-adjacent work, the pay will not be as remarkable as other companies in the private industry. I still make six figures as an L1 Software Engineer which I'm very grateful for, but it's in the very low six-figures and thus not as high as some entry level SWE positions that can be seen in any of the top private tech companies. Not to discredit my own downside, but I will say that the benefits+work/life balance do outweigh the pay discrepancy in my opinion, but if the cash value of your yearly salary is what's most important, you probably won't be the most fulfilled here. (This is for prospective entry level software engineers) If your goal is to work on something very niche and you're coming in as an L1-L2, I don't think public sector work is going to be for you. The government and its contractors have many, MANY PhDs and senior level engineers that can produce very high-quality work in very niche fields. If you come here as a new professional, your goal should be to broaden your horizons as much as possible and find a field you really want to dig into, and that's true for many companies in all honesty.