Pros
Working with top names in research. Pros depend on the project you're working on and who you are working with. Some people get lucky, others don't. I've seen more of the latter unfortunately.
Cons
Lots of people, including those in permanent jobs, complain about difficulty advancing in the agency. Postdocs and PhDs are given a lot of tedious tasks that administrative people normally do in larger institutions. It makes for a poor teaching environment. Few permanent jobs are posted. The jobs that are posted normally already have a person in mind, despite the show of interviewing "everyone", so outsiders rarely have a chance. Despite claiming to be "international', the large majority of those working there are from Western Europe. Salary is low compared to the UK or Germany. There are better places to work.