Pros
Huge opportunity for growth in the company. MongoDB is now firmly established as a company and as a technology, so the early days where people were wearing a million hats is gone as the company got more focused, but the ability to evolve your role and take on new responsibilities based on your skill set or your interests is tremendous. Generally, people just really like each other. Makes going to work fun when you're invested both in the company, the technology and also the team. Great college recruiting program. Lots of interaction between departments. Offices around the world. Regular events and parties and hackathons. Free food :).
Cons
Shifting slightly from startup to mid-size company and therefore experiencing typical growth challenges. As with most emerging tech companies, its a predominately male-led organization and lacks women in leadership positions.