Pros
Important work to be doing. Mathematica is so big that it can take on really complex and interesting work. It is very competitive on bids. Good benefits (other than pay). True 9-5 job with no expectation of work after hours. Very defined ladder for advancement.
Cons
As someone with substantial work experience and a Master's degree, a large proportion of my work was administrative work like formatting documents. I did not have an opportunity to use the skills I have, and my past experience was not taken into account in terms of what I might be able to handle on projects. I was hired at a certain pay-grade because that's where people who have a Master's degrees always start. I truly felt like a cog in the wheel. Pay was not at all competitive. I was not inspired to do my best work here.