Pros
It's a relatively small organization (<300 employees), so you get more access to higher-ups and a better understanding of how the institution works, as a whole. The culture is open and collegial. Carnegie is always hosting events and panels with scholars and policymakers. Most of these are open to the public, so there are tons of built-in networking opportunities if you're interested in policy analysis. There are five global offices (and counting), and Carnegie encourages collaboration among them, so you'll encounter a range of perspectives, instead of just the domestic take on issues. You're on what I like to think of as Thinktank Row--right between Brookings and AEI. Five weeks of PAID leave (note: almost no one actually uses all their vacay. Some people choose to cash it out when they leave.)
Cons
Carnegie's programs vary pretty widely in terms of stature and repute. The Nuclear Policy Program is among the very best in the country, while the International Economics Program, on the other hand, is a fledgling. Opportunities for professional growth with a BA degree are limited. (This tends to be the case at thinktanks, more generally.) If you're a research assistant or intern, be prepared to spend a lot of time at your desk in front of a computer. Not a con for some people, I guess, but for me personally, that could get tiresome.