Pros
It is a stable, with brilliant professionals, institution. It has a lot of money, so no need to beg to develop projects and programmes. It has an international scope and presence, so internal opportunities cover the globe. If you are not hard worker, as a contractual agent the salary is ok, if you're a performer it is not so good. If you are proactive and have a minimum of human skills, you really have the opportunity to make a difference in global development issues, because of the size and financial weight of the institution. You can also develop amazing projects and initiatives, given the previous factors, gathering a large scope of different actors around a common defined goal. A perfect institution to be if you have a large familly of your own, given the financial package offered, and if you look for a good balance between your personnal and professional life!
Cons
The administrative part often takes over the technical one. Depending where you work you may be considered and respected differently according to your contractual status. Equity is a concept that has not been mainstreamed into the institutional mechanism as well as meritocracy and even sometimes good governance. We are still waiting for the implementation of the 360° evaluation mechanism, which is not usual practice in the UN for instance. Given the size of the institution it has a hard time keeping up with innovative and fast evolving best practices and even a harder time to link policies taking into account those innovations with field projects. It suffers from it size and somehow political character, not sure the creation of the EEAS will solve or worsen the problem. For implementation it relies too much on a system of "framework contract" which keeps competition at its lower level and is often linked with an equal level of quality in the end results. Many flaws to correct, but I am confident the institution is moving the right way, particularly with the "Agenda for Change" recently adopted. But the evil is in the details, let's see how the implementation goes...