Pros
The actual work here is very interesting. As a systems engineer, I couldn't ask for much that's more exciting than helping build a database. Peers are very intelligent and care is put into making committed code as good as it can be. I've learned so much on the job here.
Kontras
Management is unskilled and un-trained. Some people who work here are jerks, and are allowed to keep being jerks as long as they produce quality code. This drives away other talented, friendlier engineers. The culture is unhealthy in promoting quality/quantity of work over most else--interpersonal relations, work-life balance, and personal sanity included. Diversity in age, race, gender, religion, undergraduate college, and many other vectors is sorely lacking. Female employees, especially engineers, are treated unfairly no matter how much MongoDB pays lip service to this cause.