Pros
I've had nothing short of an outstanding experience in my time so far at Flatiron. The company is trying to solve really challenging problems, and does so while infusing its employees with freedom to roam, explore and expand their technical skills. The passion and drive are self-evident as you walk throughout the office-- the people you work with are inspiring, and will take the extra time to explain to you why you should be excited as well. The vast, vast majority of people I have encountered here seem to embody genuine kindness, which is hard to find in the workplace due to inherent bureaucracy and work politics.
Cons
The working hours are more demanding than your average start-up (~10 hours a day) but I think that is normal for high performing young companies that are scrappy and trying to prove the resilience of their business model. Also, SO many delicious sweets around the office so difficult to eat healthy (first world problems...), but also many great fruits available.