Pros
From a software engineer point of view, you get the opportunity to work and learn lots of different software stacks. Backend services are usually either in Java or Go. The frontend is written in Ruby and Javascript based frameworks. We also have a handful of Android and iOS apps. Whether you want to focus on one area or contribute to project across the board, Square is a great place to work! My peers are amazingly talented. I think I have a good work-life balance. We also have a culture of pair-coding, which I think is great. It prevents procrastination, encourages the exchange of skills, and helps ensure more than one person is really familiar with the codebase. You can read more about Square Engineering: http://corner.squareup.com
Cons
Square is still a rapidly growing company which comes with pros and cons.