Pros
I joined Cooper along with my colleagues when they merged with our NYC company. As I got to know the company, the people, the work, and the culture, I was consistently amazed at their focus on quality and integrity. The Cooperistas are an amazing bunch - brilliant, creative, warm, and always up for some silly fun. Plus the firm's reputation in the industry is stellar - so clients come to us already psyched to partner. Cooper is a place for young designers to learn from the best and for more senior people to participate in and innovate the practice of design at the highest levels. There are not many design consultancies that can offer the same caliber of experience.
Kontras
Only the typical difficulties of working at a small consultancy: some unclear corporate policies (although this is getting better), not as much transparency into the state of the business as you might get at a larger firm, the occasional challenging client...