Pros
I've been an engineer in NYC for the last 6 years, and this is by far the best shop for which I've worked. There's that "honeymoon" feeling you get when you start a new job and coming to work for the first few months is actually fun. I still get that here after 2+ years.
Relaxed culture -- they encourage their employees to express themselves, this means no having to feel like you are one person outside of work and a different person at work. You come as and are accepted as who you are. The culture is a product of the employees, not something management dictates from the top. Everyone is able to add as much or as little as they want to it.
Strong team -- everyone strives to make everyone else better and quality of work is high. You will learn from others.
Flexible vacation policy -- get your work done and take the time off that you need.
Upper management is very attentive to everyone, regardless of position. You can get an audience with any of C-level management anytime and they take feedback seriously.
Transparency around performance -- you know where you stand regarding your performance and are encouraged to speak up if you ever do not.
Cons
As with any successful startup, you must be flexible with process change, organizational growth and office location changes as the company progresses through its early stages.