Pros
Working at the Gates Foundation provides an opportunity to be on the leading edge of work in your field with very smart colleagues, generous salary / benefits, and immense resources. People are, by and large, there because they're passionate about what they're doing. The Foundation is working very consciously to improve both their external and internal reputation. The connections you make while working there will take your career in amazing directions.
Cons
Seattle culture infiltrates the Foundation. There's a lot of talking and there can be annoying turf issues. Expensive outside consultants are used too often and consultant-speak unintelligible to normal humans is ubiquitous (e.g. "boil the ocean", "surface learnings", "convening"). It is a family foundation, too, which means that an extraordinary amount of energy and time are devoted to getting and keeping the "co-chairs" engaged in your work. If they don't get or agree with the direction of where you're going, it doesn't happen. There is a definite bias toward technology solutions, even where they aren't appropriate. And it is easily the most narcissistic place I have ever worked. "Tell us more about us! Tell us how bad we are! We want your open and honest feedback!" My biggest frustration is the lack of internal mentors or conscious career development. That and "the matrix"-- which alludes to the number of people you need to involve in anything you do. It's been humbling. Know that it is not a "dream job". The best way to get there is to be great at what you do in your field. I ignore anyone who hits me up on LinkedIn because they've "always wanted to work at the Foundation!". That starts happening a lot once you begin working there.