Pros
Fun. Fun. And lots of hard, meaningful work. They give you goals, and you figure out ways to achieve them. The people here are so smart and so kind; I've had lunch with a different employee every week, and I've never been told a flat out no. And when I needed help on something, everyone was willing to help at least at some point in the future. It totally changed my career goals: Waterfall made me realize that the startup workplace is something I love and I wish to pursue it in the future. And it doesn't hurt that they are generous enough spend tons of money on culture, which makes coming to work exciting and fun.
Cons
This is a startup. They don't have everything figured out. For example, the person working on a project that I was involved with left. There was no concrete transition plan and it took a bit of effort to try and coordinate where the necessary resources were located. That being said, the process went by smoothly. Don't expect to come here and have only your work laid out for you. It's a startup, and that means that you are going to have to take care of things outside of your job description for the best interest of the company. Personally, I think dealing with that kind of challenge helped me grow immensely as a person and as an employee.