Pros
Unique development shop that treats employees with respect, and emphasizes personal growth. Very flexible work environment with an unbeatable work/life balance.
Duo is a Drupal-only environment, and they stress continual education for all devs. (At the time of this review, Duo has the highest number of certified Acquia developers in the Midwest, and they really push developers to grow.) Management is open to new ideas and viewpoints -- in fact, the company allows you to make the job what you want.
Cons
Can sometimes feel too informal, and --although the shop is going through a strong growth phase -- they tend to be a little loose with their practices. Also, they sometimes are too forgiving with employees who wouldn't last at other companies -- especially on the PM side.