Avantages
The are a lot of very smart people. It's a professional and friendly environment to work in. Most people I've met are truly just trying to do the best they can and produce the best product for the customer. The benefits are unmatched in some areas. For example, health insurance is 100% covered. There are no premiums. The company provides a fully funded HRA for each employee that covers any out-of-pocket medical expenses. I can't tell you how nice it is to not have to worry about how to pay for a trip the the ER.
Inconvénients
Stack Overflow is growing fast, and so there are certainly some growing pains. Cross-team communication and requirements gathering can sometimes be difficult, but the company is always iterating on ways to improve communication.