Pros
The team is incredibly talented and a lot of fun to work with. At the same time, they are pretty humble and always willing to help, so it's been a great opportunity for me to learn a lot of new skills and technologies. What always amazes me though is how well people get along cross-team. I think it gives a really strong feeling of unity across the company as a whole. Hopefully it stays that way even as the team continues to grow. In terms of the work, you definitely get a lot of responsibility and are encouraged to take ownership of what you work on, which is a plus for me. The technical challenges are also pretty interesting. Management is also pretty responsive and constantly seeks feedback for things to improve. It's been great seeing a lot of that feedback acted on.
Cons
The work can be demanding at times. It's the flip side to having a large impact and a lot of responsibility at a smaller company. Taking time off to recharge and spend time with family is encouraged, but it doesn't change the fact that sometimes you might need a long night to get things done. Personally I'm ok with it, but it's not for everyone. Definitely something to consider when joining any start up. The perks & benefits are solid, but they aren't as nice relative to those of some other technology companies in the bay area that I hear about. Not a con for me, but wanted to note it since it might be for other people I guess.