Pros
Square is an amazing place to learn and grow. The speed of change sparks affords new opportunities on a very regular basis. Even as the company has grown and gone public, the culture is still very horizontal and executives and higher level managers are still very approachable and open to new ideas. Perks are great - flexible vacation policy, good food in the office, plenty of snacks, coffee bar, and a beautiful office. The CEO has strong morale convictions and this clearly guides his leadership. It's very refreshing to have someone this authentic heading the company who speaks their actual opinion about social issues. Even when I don't agree with him, it's nice to know it's actually his opinion (opposed to something his legal team drafted up).
Cons
A lot of these issues have been fixed, but due to the rapid growth there are still some systems issues and managers that are in over their heads. Most of these Managers have been replaced (or moved to a more appropriate level), but some remain in higher level positions who lack the necessary experience in critical areas which is still causing some pain. The HR team is understaffed so they are not in a position to offer much support, but this is currently being addressed. The CEO's political beliefs create a somewhat insular environment, though I'm sure most tech companies are like this. It would be a challenging place for someone with conservative political views to work at.