Pros
Pay and benefits are fantastic. These include six months of paid parental leave (although it used to be a full year) and a contribution to one's retirement fund of 15% your salary. In effect, this means that compensation is 15% higher that the salaries you see quoted. Also, if you work and live in Washington state, there's no state income tax. They have some very talented and experienced people at the foundation, and the mission, of course, is a very fulfilling and meaningful one. The foundation also has a very good reputation in most places, so it's a good line on one's resume, and contacting people with a foundation email address helps a lot in getting responses.
Cons
Ultimately it's Bill and Melinda who make the decisions. They are very involved in the work of the foundation, and _a lot_ of effort goes into preparing for presenting work to them, and it can be quite a political place. There's also an annual strategy review for most teams, so a lot of energy goes into preparing for that every year. The culture is very different from team to team. Some are amazing and collegial; others are absolutely poisonous, so ask around and have 1:1 conversations with people there before joining a team.