Pros
The people I worked with were the best takeaway from my experience. Although the office environment was fairly deadly (undrinkable coffee and no natural light in many areas), the passionate and intelligent staff made coming to work enjoyable (when people actually came to the office instead of working remotely - it was often very empty). My takeaways: I learned a lot about the humanitarian sector and have some great friends and connections. The field staff in international offices are also inspiring, dedicated professionals.
Cons
The organization is changing. Over the years I worked there it grew quickly, which isn't necessarily a bad thing, but the headquarters office is turning into more of a corporate working place in that there are questionable donors supporting the organization and I think there are some ethical issues there to be unpacked. There was no transparency about salaries and people who had been there for years are earning less than people from the private sector who come in and don't have the same skills or experience. As far as my personal experience goes, it really all depends what department you are in, who your supervisor is, and if they are able to effectively manage you. I was poorly managed and the situation only worsened. I have actual trauma from working there. My role was at times very unclear, there was internal competition, and I was not given the support and guidance I needed to improve upon things after getting negative feedback. I was ignored by management when asking for guidance and misled by HR internally.