Pros
Flatiron is an amazing, special place to work. I've been here two years now and can't imagine leaving. Aside from being a part of the incredible mission, they really live up to the values on their website. The founders are accessible and thoughtful about people and their happiness and development (lots of management training, fair, thorough performance review process etc). Decisions are made based on un-emotional logic and cross-functional feedback rather than top-down bureaucracy. People are open to being wrong. Change happens quickly. I was extremely surprised to even see some negative reviews on here as they don't reflect the overall feeling at Flatiron -- people are happy. Very few people have left and the engagement survey scores (which measures things like happiness) were off the charts!
Cons
Lots of snacks, but wish there were more unhealthy options :-) It's hard work, though people are reasonable in their expectations of you, e.g. very flexible hours, unlimited vacation.