Pros
Mathematica has a really strong culture of excellence, objectivity and rigor. Many of the researchers are leaders in their fields and the company works with top-notch clients doing cutting edge research. I'm impressed with how management is really changing the culture to meet the current times -- they are flexible with work schedules, open to new ideas and approaches, and embracing technology. This isn't a company that jumps on the "new" thing. Rather they jump on the right thing for the kind of work they do. There's also a real infusion of new talent and energy. A third of employees have been with the ocmpany less than three years. This is balanced by the fact that many employees stay and have long productive careers. It's not unusual to be 20-year veteran.
Cons
There are many smart, accomplished and talented people. You don't advance "just because." You have to genuinely differntiate yourself and show initiative. Because it is a culture driven organization there aren't always hard and fast rules or systems -- that can be challenging for people who are shy or have difficulty with ambiguity.