Pros
Everyone is collegial and smart. You have complete flexibility over your hours and location. Some of the projects are fairly impactful and prestigious - particularly the federal healthcare work. 401K match + ESOP contribution rates are good.
Cons
The pay is not competitive with the market. Pay increases typically don't keep up with inflation. Promotions take ages (4-6 years on average to get a promotion.) There is no paid family leave - just a weekly supplement on top of your state disability benefits give you, and the supplement only lasts for the duration of your state disability benefits. There has been significantly higher churn in the past few years. If you're in one of the businesses where the company is well-established and competitive (like federal healthcare), you can have a decent experience - the pay will still be low, but it's stable, impactful, and not very stressful. It's much more challenging to be in one of the business areas that Mathematica is trying to establish a footing in.