Pros
Move fairly quickly here at work. Solve interesting distributed systems problems. I've been at google, other similar startups, and other non-sfbay companies. Square is my favorite because of the real problems we are solving, the focus on design and building the right thing and helping merchants. The upside if we succeed is very big - everyone in the world does commerce daily - we want to make that better.
Worked here a long time as an engineer before trying management. Engineering managers are all engineers, which is a very good sign.
Cons
Salary is lower than google, but that's what you'd expect when you trade equity for cash.