Pros
There are so many things that need to be done with the high growth rate of the company that nobody is pigeon-holed in just one place or one just one type of job. Flexible hours and vacation are great but underused by many. People are happy to hear your ideas, and discuss them; as an open source company there is no limitation to what you can do on the internet with mongodb.
Kontras
Like any growing company there are more challenges along the way, and these include: lack of organization responsibilities, definition, and roles, trouble with building effective communication channels internally (betweens groups and depts), lots of new employees which need to learn/train before being effective, holding work quality high, professionalism, over-worked and under-acknowledged employees, as well as providing structure for professional growth internally.