Interesting work; few opportunities for advancement (or pay increases)
Pros
Excellent benefits. MITRE puts in an almost-unheard-of 7.5% automatically, and with matching will go up to 10% (they take one year to start matching, but you're fully vested from the get-go). Net result is that you're putting in 12%, and they're putting in 10%, for a total of 22% per year. They also have student-loan repayment up to another $1800 per year (although only for 3 years). They also have subject-matter experts in almost every topic you can think of. So you can save a lot of time by finding an expert to look over something, or bring you up to speed. They have continuing education opportunities through the internal "MITRE Institute". Depending on the group, they can be extremely flexible about working hours. Work-life balance is solid; you won't be criticized for putting in your 40 hours and then going home.
Cons
Promotions beyond Level 3 are rare (as in, one person per department per year from 3 to 4, one from 4 to 5, etc). So functionally they're awarded largely by seniority, as opposed to merit. Pay is tied very closely to salary band: Level 3 (Senior) is centered at $110k (+/- 25k), Level 4 (Lead) is centered at $140k. Management convinces themselves that they're "paying above market", but that's not really correct. While nominally you're paid a bit more than in another company with a title of "senior" engineer, the truth is that in those other companies, you already would have been promoted. So relative to your actual job performance, you can be severely underpaid. This sets up a perverse incentive within the company: the most capable/ambitious employees are likely to find another job, while the least capable are likely to stick around in hopes that they'll at least get promoted based on seniority. The company certainly has high-quality people... but less than if they used a different promotion scheme.