Pros
You get to work with a complex and growing technology and vast array of clients. Your next job after MongoDB is virtually guaranteed. Between your client list and the people you meet at conferences a year here is a ticket to anywhere.
Kontras
A year here may indeed lead to a ticket to somewhere else. MongoDB has a very demanding culture and is really looking for "Google-caliber" brains. The problem is, their comp package is nowhere close to Google and others. After recent valuations, new options packages are going to be very low value, they are well past "startup" phase. The biggest challenge with MongoDB is actually the CTO. Despite its popularity, the MongoDB code is really sloppy and suffers from some poor technical decisions that may haunt it forever. If you're an engineer, your inevitable job is to support the product. That means providing support for the sloppy, undocumented code and the poor technical decisions. This makes the job extremely difficult.