Pros
At Box you never find a shortage of things to work on. You can continue to grow your skills and expertise by tackling new and different problems more than you can at most other places. In addition, the people you work with are some of the best and from a wide variety of backgrounds. This means you even learn by listening. And since Box is still a young company with problems and situations that are still unsolved, you can use the skills you learned here or elsewhere and leave a mark on the company. Something you work on will very likely be lying the foundation for future engineers and users. It's up to you to leverage that. The compensation is pretty darn competitive for the area.
Cons
The compensation could be higher and internal mobility isn't exactly where it needs to be (they are working on it to their credit). And sometimes the opportunities you get means that you will be working some late hours. I've made it a goal to at least leave when there's sunlight at least twice a week. And as expected with a startup mentality, there's a bit of engineers being oncall which can sometimes lead to annoying weekends.