Pros
You get to work for the poor and it is a diplomacy, high-level, work that has an impact on millions of people. You also get to work with some of the greatest experts in the field of development, and can learn a lot. worked in a rare field and was lucky to get some of the very cool consultancies, got published all over the UN and web. This of course felt great. I am thankful for this experience, but would not be thrilled to go back.
Cons
The percent of inefficiency is too high. Too many people working there couldn't care less, and neutralize the work of the very commited, hard working ones. It is very nepotistic and political, and only 10% of people who get it, got via regular hiring channels. Jobs there are rather 'inherited', or political (for the very high level). The quality rarely gets awarded or promoted. If you are over fifty and given up on your ideals, this is a good place to work. Otherwise, you may find yourself in the situations when you'll feel the guilt for getting paid from the development money, and not feeling it really helps.