Pros
I worked with some of the smartest and most creative people in the ad industry here. A pretty good balance of data-driven vs. creative-driven work. The tension between creative teams and analysts was pretty strong, but I thought it was a good thing for the quality of the work. There was a good amount of learning and development opportunities, a relatively flat organizational structure (could walk into most offices without a meeting being scheduled), and some opportunity for career growth. However, some people definitely felt the last point (opportunities) were few and far between. Work-life balance was good and the agency participated in the community and volunteered often.
Cons
Like any agency, some managers were better at their jobs than others. Raises and promotions seemed to come too infrequently for many people, leading to an unnecessarily high turnover rate. There was also a lot of restructuring for seemingly political reasons. But, for more junior-level employees, I still think this is a good place to work.