Pros
I've worked at large firms (6000) and small (50) and seen the different kinds of politics and red-tape that slow down most companies. This is the most open and pragmatic place I've worked. Focus is on problems and solutions, not turf, not authority, not blame, not work-avoidance. You want to have an impact or solve a problem here? Just jump in.
Cons
The size means your day-to-day is going to be unpredictable. Many people wear many hats. That's part of the fun and opportunity - but it can also be disorienting and frustrating if you like doing one thing without interruption until it's done. You need to be able to change gears quickly, or this might not be for you.