Pros
The rate of growth is exciting, the company personnel is surprisingly tight-nit, it seems good work is well-recognized, and opportunities to grow and gain responsibility are abundant. Another current (probably temporary) benefit is that most people rarely (if ever) need to work over normal business hours, which makes work/life balance very easy. Biggest pro for me personally is seeing the immediate effect my actions can have (for better or worse!). This adds both pressure to succeed but also pride when things go well.
Cons
There's plenty of growing pains in processes, people, etc. This isn't really the type of job for someone who doesn't want to work hard and can't handle pressure. Because they're still small but growing, compensation is comparably low, though it seems trending upward.