Great people who care and want you to succeed. Good strategic planning process and unique proprietary models. Great resources to do a good job. Teamwork is usually very good.
Cons
NK is primarily focused on BtoB clients (although great brands), I wish they had had more diverse clients, particularly consumer clients. I left to work on a consumer, retail account (QSR) and it wasn't all as glamorous as I thought it would be - thought I did better work at NK.
The people are awesome, and it has a strong culture of doing good work. I especially learned so much about the business, how to think strategically and more. I left to work on different kinds of accounts, but I really miss NK. They treat employees so well, and want you to succeed.
Cons
Lack of summer hours; lack of different types of experiences to help build my career. Was told that I needed to work several places to "improve" my resume/long-term career pursuits.