Pros
One of the top jobs for an economist, it gives you the opportunity to work on a vast range of issues across a very diverse range of countries (basically the whole world). Very stimulating, always at the frontier of policy and research, great working place with truly international, multi-cultural staff.
Cons
For being a BIG international institution, I find the IMF to be quite agile. But it is still a BIG guy, with internal bureaucracy sometimes taking a toll on the otherwise very dynamic nature of the job. Work-life balance is on the lips of the whole management ... but actual progress on this objective is a different matter. Depending on the area you work in, you might find yourself working very often on weekends and in the evening (especially if working with countries at the other side of the time zone range).