Pros
This is hands down the best job I've ever had. It's wonderful being able to work on an fascinating and difficult problem with a team that's up to the task. Management clearly ambitious, but also realistic and humble when being humble makes sense. Both founders are themselves exceptionally technically talented, so there's little management-technical friction. The aim first to solve the difficult technical challenges we have set ourselves, and to do so in a responsible, healthy, and outward-looking way. To that end, the team is talented, hungry, and welcoming. Also, it would be remiss of me to not mention the food, which is spectacular. I'm slightly in love with the chef.
Cons
There are always downsides with working at a startup, and this is no different. Uncertainty, rapid change, and fighting fires are all downsides, but pretty essential to a startup this ambitious.