Pros
Mathematica is a good place for Associate/P1 -level staff to gain experience in a variety of job functions and across a wide range of subject areas. The people are generally kind and intelligent. The company has systems in place to document and disseminate SOPs, and is generally fairly organized. I would recommend this position to people who are very resilient to stress, excellent at advocating for themselves, able to manage 4-6 discrete projects simultaneously, and willing to work for less than the market value of their labor.
Cons
There is a significant culture of overwork at Mathematica, and the pay is not commensurate. Associates are pressured to take on increasingly high levels of responsibility (serving, for example, as Project Managers for portfolios cumulatively worth tens-hundreds of millions of dollars), but they are only paid $55k-65k. Unpaid overtime is normalized. Associates, who are hourly non-exempt, regularly work 50+ hours/week but, like everyone else, are basically required to document only 40 hours on their timesheets. As another reviewer put it, there is a culture of working more hours than are billed to the company's clients. Many staff are spread thin between their projects, some working on 6+ at a time and at >100% LOEs, and that exhaustion sometimes leads to corner-cutting on projects.