Pros
As others have said: great people. And this is important, because this place recruits smart wonderful colleagues, people that are truly brilliant and, when they're not directing a project, are the best humans to have as colleagues you'll ever find.
Cons
You have to take the good with the bad. For some, the bad is too much. For others these points may not be that bad. 1. This place breeds micromanagement. The processes are specifically designed to take out all critical thinking and ownership. But, if you get a manager/project director willing to go against "the Mathematica Way" you can have a really rich experience. Goes back to the ultimate strength of Mathematica is the people, not the processes or internal methods. As a result, people with strong capabilities have their growth stunted by a really bad project management ethos and because of overincentivizing adherance to the Mathematica Way. 2. It is impossible to respect senior leadership. The CEO lies and then when he's called out, he fires the staff, yells at them, or has HR intervene to downgrade their performance review. He talks about the "Mission" but anyone who has worked at a mission-oriented company can see that this is lip service. The subterfuge is demoralizing. 3. It's every person for themselves (depending on the technical area). It's just a fact that you have to look out for yourself. Looking out for the good of the team, unit, or company will only lose you nights and weekends while the senior execs pocket all that revenue you just generated.