Pros
Affirm is innovating in a big space by issuing loans in real-time and providing top-notch loan servicing. The company is still small (~30 people in the tech organization). These aspects make being an engineer very challenging and rewarding--we are small enough that every contributor needs to learn how to do a lot, with many developers contributing to the full stack. Each of us need to develop enough awareness of the lending industry and its regulatory components to maintain a compliant lending system. This leads to a serious engineering culture with a big emphasis on automated testing, as bugs in the financial space can have dire consequences. Everybody on my team is extremely intelligent and great to work with and learn from. Overall, Affirm is a great place to work as a developer, especially one who enjoys high stakes challenges and developing a large breadth of software engineering skills.
Cons
The biggest detractions to working at Affirm relate to the biggest benefits; since the stakes of what we are doing are so high, at times, things are stressful and require us to be extremely focused on our work. In short bursts, work-life balance can be very tilted towards work, although there's nothing quite as satisfying as deploying a financial product that hasn't existed before. Also, the company still has an unlimited vacation policy, and it's understood that folks need a break after putting in tough hours for a sprint. While I've only once in 2.5 years at Affirm had a manager actively tell me to take a break, whenever I ask for some time off, it's never a problem. Work-life balance is achievable at Affirm, but it requires the employee to be proactive about taking care of herself.