Pros
Passionate, talented people. Leadership cared about the staff's career growth and tried to find interesting projects suited to your strengths/interests. Everyone was passionate about the work. I think the client relationships were built from that passion. I felt motivated and supported while there. The CEO was always very transparent, and I appreciated that. They try to get the best work out of you and you learn from some of the best. Generous bonus structure and ability to move up when you're ready (don't need to wait for the person above you to leave, like at the holding companies).
Cons
Small agency culture isn't for everyone. You have to enjoy multi-tasking and doing things you've never done before. Sometimes that can be challenging, though.