Avantages
Nice and relatively low-stress working environment. Solidarity of mission. Good people. Decent benefits and lots of PTO. If stable, long-term employment is your goal without too much variation in what you do, it is a great place to be. Lots of people make this their final job and take it to retirement, so if that is your goal, then it is wonderful.
Inconvénients
Office politics are moderately to high. Sometimes it is difficult to use the generous PTO because of how much there is to do. Promotions seem more motivated by who you know rather than what you know, so advancing your career is difficult. Your best chances at being promoted are someone leaving, but that doesn't happen very often. Odds are that you will be doing the same thing on your first day as you will on your last.