Pros
The company hires people with advanced degrees. Most people have a private office, which is nice and helps productivity when you have a lot of writing tasks. The pay is decent (though not adjusted for office location). The benefits are decent. Mathematica is well respected and can serve as a good stepping stone to a different job or different career. In principle, Mathematica doesn't have a set number of people at each level, so you if you have the skills, you can be promoted to the next level without waiting for someone to retire.
Cons
The company is very hierarchical in how it is structured. Communication from management can be slow, and messages are usually delivered long after decisions have been made, and without input from staff. It's also sometimes inconsistent - you'll hear different things from different people at the same level. Oftentimes there's a lot of petty one-upmanship with people trying to undermine you. Superiors not always understanding/empathetic towards employees. Most people are nice, but many harbor resentment and envy, especially when they feel threatened - that is, they're high up and worried a "cheaper" person will make them obsolete. This problem has more to do with management than those individuals per se, but the management structure and incentive system causes several senior people to feel that way. The company has very questionable promotion practices. In terms of hiring, there is little diversity, despite the office locations in cities with diverse racial and ethnic populations. At the professional level, the company hires few people from underrepresented groups. It's also curious that the few non-whites they do hire take (on average) much longer to get promoted than their white counterparts (at all levels, junior to senior, and all divisions, health, human services, and survey). The HR department is friendly but their staff don't really do anything/advocate for you when you have a problem. Training is haphazard and less than satisfactory. It takes forever to get work when you first join the company, especially "interesting" work. Expectations for junior staff are low in some respects (in terms of analytical ability, leadership ability) and high in others (going after work, remaining billable). Most importantly, opportunities for learning and developing career skills are very limited if you're a survey researcher. Also, opportunities for promotion are limited in reality and don't often seem based on merit.