Pros
It was great to work for a donor institution. People always responded to my emails and calls. I contributed to the decisions on how to spend millions of dollars to support valuable organizations in Latin America and the Caribbean reducing poverty. I perfected my Spanish and learned Portuguese in the library. They had unlimited resources in terms of subscriptions, research support, experts on gender, environment, Bolivia, etc. It was also a great work-life balance. The culture is for long lunches, afternoon coffee in the atrium, lots of travel, yoga in the gym, movies and music in the concert hall, and long vacations.
Cons
Very political. They seem to be constantly reorganizing. Seems like there's a lot of dead weight (I once walked by a room where a woman was taking piano lessons...!?) The bureacracy can be really overbearing. 2/3 of employees are consultants, and it seems nearly impossible to become staff as an American. They have quotas on how many people they can hire from each member country and since they are based in DC there are a lot of Americans in the city's international relations grad programs interning there and trying to get in (SAIS, Georgetown). There's lots of competition, they can replace you in a second. US tax laws also make Americans pay 30% of our consulting salary in taxes, so we cost the organization more (or you take the job and are living in a crappy group house while your European colleagues buy condos and go out for fancy dinners every night). No benefits. The "Research Fellow" position is just a crappy way to pay you less with no benefits and no job security, and we all overwork ourselves since we're fresh out of college (and hoping that will mean they will hire us . . . they won't). Try to get a JPA at the World Bank instead if you can. They pay for health insurance, taxes, and even some retirement savings, or so I've been told.