Pros
Square is a technology company that sells a "real" product. Our many many merchants depend on us to help them run and grow their businesses. When our merchants grow, we also grow with them. The incentives are perfectly aligned for us to help them do well. We don't need to sell ads based on their information, and we're not selling acorn points or whatever. I work with incredibly talented people who have a wide range of experiences. We have a lot of fun at work, but we get our work done. Since Square has such a broad spectrum of engineering work, from hardware, to firmware, to back end services, to security, to infrastructure, to front-end (think Ember/JS/CS), to mobile (Android and iOS), there are so many opportunities to learn and to try new things.
Cons
It's taken us awhile to get our product vision/road map in place, but I think we're now in a good place. We're not as extravagant with our perks as other more cash-rich tech companies, and the (free) dinner can be a bit hit or miss. The company has almost doubled while I was here, but that's simply necessary for us to grow our business.