Pros
I've never worked in at a company where I truly am energized and excited to come to work every day. The mission of the company is really fulfilling and you're surrounded by incredibly smart people. Everyone here is collaborative and there is no competition - everyone is committed to achieving a common goal and people are appropriately recognized for the great work they do. It's really an amazing thing - the culture that's been created (and kept intact as we rapidly grow) is really special and makes working here so much fun. Even though there's ~600 employees now, it still has a start-up vibe so there's plenty of opportunity to grow and be challenged. Management really trusts and empowers the staff to make decisions and cares about the results of your work, not how many hours you put in every day. They truly embrace a flexible work schedule and really respect people's personal lives.
Cons
The start-up environment does have it challenges as there isn't always a set way of doing things, so there's often a fair amount of figuring it out along the way. Depending on your personality, this can be a good or bad thing. Some people are not totally comfortable with the flat structure (ie, no org chart), but it's also a great opportunity to work with people at all different levels to learn from different perspectives.