Pros
I chose to apply for a position at AI2 because I wanted to work on something "important" that could potentially be my "life's work". After six months on the job, I feel that I made a great life changing decision. I have never felt so humbled and fulfilled at work as I do at AI2. The caliber of my co-workers motivates me to continuously produce my best work. Aside from the work being great, the culture is also great. People are encouraged to take leadership opportunities via our "take personal initiative" policy. The company director (Oren Etzioni) is committed to being transparent and enabling employees to manage their own work while holding us to very high standards. I think this works very well because of the caliber of researchers and engineers we have hired. It just feels like the right way to run a company of world class knowledge workers. Also, the company is committed to work/life balance. Vacations are encouraged and workaholic hours are not.
Cons
If you are used to being the "superstar" performer (as I was), then AI2 will be humbling. I actually consider that a pro, but I imagine some may not.