Pros
Propeller is made up of a some incredibly welcoming and super smart people, who are never hesitant to lend a hand or have a chat. They make you feel like part of the family from day one. Leave your ego at the door, there's no petty competition here. Everyone is genuinely supportive of each other and you really get a sense that everyone is working towards the same goal.
Cons
Fairly loose work structure means a lot of figuring things out as they go. Be prepared for lots of experimenting and changes as the company finds it's legs and figured out the best way to get things done. Flat hierarchy means you're expected to step up, work independently, and be self-driven. The culture is friendly, but at the end of the day, you have to be driven, passionate, and good at what you do.