Pros
The people are smart and dedicated to international development. The projects have tremendous impact and provide countless professional development opportunities for motivated employees. You will learn everything there is to know about bidding on and managing USAID contracts. With the ISO certification, there are clear processes outlined for all core business functions, making learning new things much easier. Also lots of internal training on technical and management topics. Being 100% employee owned means they are not likely to be bought out by another firm, and employees can make money when the company succeeds.
Cons
A lot of your experience will depend on your supervisor, though that is true anywhere. To really succeed, you have to be motivated. A sit-and-wait attitude won't get you very far here. If you don't like the business side of development, this isn't the place for you--there is a constant focus on winning new bids, and everyone in the company is expected to contribute.