Pros
Management that truly listens. A culture of constant improvement, with smart and passionate coworkers. Working on a good product at a company that cares about it's customers and community. A dynamic and quickly changing environment. A reasonable work/life balance. Unlike some reviewers, I find the work life balance totally acceptable, but it does require require some amount of personal responsibility. Decide how much you're willing to work, and then do that. I tend to work 40 hours a week, and take several weeks of vacation a year. I encourage my team to do the same.
Cons
Square is still a young company, so it's still figuring out how best to work. There isn't always the same structure you'd expect to find at a more mature company. We have a high standard of quality for all of our work, which can be both a pro and a con.