Pros
Committed staff, with a little motivation you can provide a valuable service, great place to learn how international NGOs work (and don't work!), good work-life balance, some (though a minority) of excellent managers, positive working relationships at the junior and mid management levels. The most interesting and impactful work is done in the public policy and humanitarian teams. In third, the Communications team is also trying to be more innovative.
Cons
Top level management (the CEO and all the VPs, with the exception of the humanitarian lead) is perpetually unable to work together to get things done. The CEO has generally surrounded himself with VPs who will support and defer rather than challenge him, let alone one another. Avoid working for the Membership and/or Strategic Impact Team (which focuses on international development) as the leadership is uninspiring and the work as a result is very constrained. There isn't great room for growth or increased responsibility. Though you may get promotions and raises periodically, the actual job rarely changes. That said, since you work with so many NGOs, many staff get the chance to get better positions with members.