Pros
Amazing product & great culture: positive, constructive, kind, motivated, talented. Everyone is highly intelligent, both intellectually and emotionally. They protect this valuable culture and take it seriously. It's such a healthy environment that stress is low and productivity is high. I love coming in to work every day!
I've worked for a handful of high-profile startups and this one is as good as it gets.
Cons
I can't think of much... the office gets a little bit crowded? No 401k match (yet). Other than that, I can't think of anything else.