Pros
The salaries aren't bad, and there are still a lot of knowledge and talented staff there that try to maintain the old, collegiate environment that once made Mathematica a fun place to work. Unfortunately these people are underrepresented in senior leadership and management roles.
Cons
There are a lot of trade-offs you make for that 5% you get in stocks each year based upon your salary ... First, the other benefits Mathematica offers are inferior (at best) compared to most other companies. This includes the super expensive health plans, vision, and dental options the company offers that don't provide much in coverage. This is especially true for the dental and vision plans. Mathematica also *does not* provide full pay for maternity leave for *any* length of time. The most they offer is 60% of your pay for a month or two if you're a woman. If you're a guy, you don't get any pay at all for paternity leave, and can only rely on whatever vacation time you have for pay while you're on FMLA. The politics are the biggest con by far. Promotions and raises do not seem to be based upon merit or ability so much as company politics and who in leadership is pulling for you. I've seen project leaders drive a project right into corrective action only to be rewarded with a promotion months later because their supervisor and/or VP wanted it to happen. Despite being "Employee Owned" you don't really have much say on the projects or type of work you get assigned, particularly if you don't have a supervisor with the ear of someone in a senior leadership role, or a supervisor that cares. After the recent reorganizations, people no longer have supervisors located in the same office (or even state), which doesn't help matters.