Pros
Continual momentum as a growing company, with the infectious energy only found at a startup. A thoughtful and selective hiring process has resulted in a true team environment where no one is "above" helping anyone else to get the job done...even when the vast majority of the team is exceptionally intelligent and talented within their speciality. Flexibility and openness from schedules, PTO and dress code means you don't have to have a "work" version of yourself. Fast-faced, innovative and results-driven organization where autonomy and collaboration co-exist well and experiments and creative thinking are encouraged and backed by the leadership. Dog-friendly office with snacks and weekly all company lunch.
Cons
As with any startup, there's a lot of change and never enough hours in the day/resources/etc. For me this isn't a con, but it might not be the best fit for people who thrive on routine and predictability.