I worked with USAID as a contractor within a partnering organization as well as a direct employee as a development officer. So I have seen several sides of how the organization works. I enjoyed my work, but considering the locations from West Africa to Iraq to Afghanistan, I am glad to be out and back in civilized society. Probably most difficult aspects was the environments in which I was working. One can only take so much. However, I learned a lot and would do it again if needed.
Well, it is government so there are limitations to innovation. I also work on a small team, there are some aspects that create challenges. I would probably do more things together even as a small team to remind us of our purpose, to connect us with the work we are doing so we don't feel like just paper pushers.
It's a great place to work, with a few caveats. First and foremost, it is government so expect bureaucracy and slow change. Second, there are many people considered "experts" that are experts of nothing, really. So be prepared for inflated egos and views of self worth. Otherwise, the people are awesome and into the mission. Great!