Pros
For a coder, MongoDB provides interesting problems in systems programming and large-scale software engineering. Plus, most of the interesting code is open source, and participation in the open source community is encouraged. There are opportunities to learn technologies, OSes, and programming languages, and a fair amount of flexibility if you want to switch teams and learn something new. There are plenty of tech talks and reading groups to level yourself up. The company shows signs of being immensely successful over the next decade--even now, working for MongoDB is a chance to join early with a company and technology that will probably fly very high.
Kontras
The NY office is in the middle of Times Square. Over-aggressive deadlines mean some teams have to work overtime for periods once or twice a year. Even at other times most development is done urgently and with an eye toward the near-term: this isn't Bell Labs or Google. As the company makes small pivots on its way to profitability, employees can feel blindsided by major decisions made in secret, such as MongoDB acquiring a partner or making a deal with a huge customer.