Pros
Very good culture within Mathematica that made people feel welcome. Free bagels on Fridays. Mostly positive working environment, with people who are willing to help you out when they can. Celebrated everyone's birthdays within our group. Was able to collaborate with other departments on different tasks and projects. Still talk to a lot of people that I worked with at the Princeton office.
Cons
Started to really change around 2016 or 2017 - became more corporate as time went on. Offices (at least at the Princeton location) were emptying out because so many people were given the option to work from home. New people were coming in and trying to change the culture into what they wanted it to be, instead of becoming a part of the existing culture. Doing certain things to "have fun" seemed very forced at times, like they were trying to hard. Also on a side note - there was one woman in the IT group who CLEARLY has issues, and is unliked by a lot of people . If you meet her or work with her (if she's still there), you'll know who I'm talking about.