Pros
It's a more complex (and interesting) organisation than you might think. I worked on lots of cross department projects, and loved always learning new things about the organisation. I also loved working with a broad spectrum of people. There are great formal development courses, a great intranet site (really) where you can find out almost anything, reasonable working hours and perks and some very nice offices.
Cons
I couldn't believe how bureaucratic it was when I started working there. I think this has improved a little but you need to be prepared to navigate some pretty frustrating ways of working. Management are of variable quality. I've had some brilliant managers over the last few years who helped create opportunities for me, but before that I was at the mercy of several shocking ones and they appeared to go unchecked. Constant ineffectual re-organisations. Also the number of people who want to make decisions on things which are outside their span of control is quite extraordinary - it makes getting anything decided extremely time consuming.