Pros
This is a young, fast growing company with unique business and technological challenges. There is a large degree of freedom in finding the right technological solutions and a direct connection between individual performance and company performance. You will get to propose new directions and approaches to solving problems. The management structure is flat so you basically report directly to the founders or the product manager. If you pitch something which might work, you are rewarded with the opportunity to make it work. There is no shortage of new fields to explore, from design and user interaction to scaling the service, API design and machine learning. You'll work with very talented colleagues on improving and expanding a consumer service that is truly loved by its users.
Cons
The sheer number of challenges can be overwhelming. The company is still in a somewhat trial-and-error phase. This is great if you love experimenting, but not so great for long-term planning. Take a two week vacation and expect to find a completely changed workplace, with new colleagues, technologies and methods. Decisions are often made on gut-feeling and can be reverted just as fast as they're made. There is a high level of dedication, so be prepared to sacrifice some evenings, nights or even weekend hours.