Pros
I think that people come to work at Quantcast because the company gives them the ability to own and work on challenging, substantial problems that clearly have impact. The people that make up the organization are wonderful--smart, approachable, down to earth, and get things done. The opportunities, people, and upside are probably the things that are most rewarding. The idea that you are working on something big and trying to make it big is a constant driving force. It's a relatively small company, which gives people the freedom to "just do things" to solve an abundant set of novel problems.
Cons
The company is growing fairly rapidly, so there are still kinks being worked out to balance facilities/amenities with this growth. I think that the way things are improving, this will be a non-issue in time.