Pros
Square engineers are smart, motivated, and flexible. We get to work with a variety of technologies (for those who want to, at least), and there are always new things going on (technologically, business-wise, geographically). Square is innovating in a variety of ways to make things easier for our customers. It is a collaborative environment so it might be a better fit if you enjoy working closely with other people than if you prefer to sequester yourself with a problem and solve it on your own.
Kontras
Although the general management and prioritization situation has gotten better with a few really good executive hires, there are still things which fall through the cracks given shifting priorities (which up to a point is desirable), re-orgs, and an emphasis on building new things (keeping existing things running does happen, but isn't planned for by management in the same way).