Pros
The benefits are unbeatable--4 weeks of days off to start, going up to 6 weeks. If you stay awhile, your salary will be higher than what you can get outside. Volunteers are passionate about the organization. You will almost surely be in contact with project managers around the world if your job involves volunteers. Travel possibilities.
Cons
Politics, favoritism and a culture of fear. Bosses are way more worried about what their boss will think than the positive reasons for getting a task done right and on time. This culture goes right to the top. PMI doesn't practice what it preaches. While there is tons of pressure to get too much done too soon, deadlines are rarely met and quality standards are poor. As others have said, lots of firing. Don't aspire to be a manager if you want to avoid that part of PMI.