Pros
This is just a great work environment. I feel stimulated by the folks I work with (who are all clever, interesting people with great values) and actually excited to come into the office every day, which is not something I've felt everywhere that I've worked. Because it is a small company, all employees have a close, open relationship with senior management and it does feel like we all have a say in the direction of the company, which is unusual. The work is fascinating and deeply relevant, and even as someone more on the administrative side I feel like I learn about more about the world just by osmosis.
Cons
The cons that come from being an EA anywhere - long and sometimes sporadic/non traditional hours. Also, the small size of the company does sometimes lead to extra responsibilities (i.e. one person doing the job of 1.5 or 2 people). More often than not, though, this is voluntarily taken on as the work is stimulating and it's hard to not want to get involved (at least in my own experience).