I can only echo what has already been said. I did a couple of years, on and off, and I was really disappointed with what I saw. There are a few well intentioned people, hamstrung by the system, but the majority are out to get their "field time", and contacts, in order to climb a very complicated promotions system and so secure their job in New York. The amount of money spent on "support services" is fairly outrageous. No one (that I saw) was ever vindictive to any of the locals, they were just being used as part of a system, but some nationalities treated them deplorably. In one case the whole of a countries armed forces working there were sent home, their contract "came to an end!" It was never publicly stated but everyone knew it was because of various events that occurred involving treatment of the locals. A lot of the poorer countries around the world get involved in UN operations so that it earns them brownie points and kudos in New York and they also get a refund on their donations to the UN. Call me cynical or naive, I know I was disillusioned.
If you fly at all it is just for a contracted company or military (quite a number of those globally) and they are not necessarily only flying for the UN. As already said some of the flying is interesting, standard stuff if you've been in the military, and you'll end up with some good stories and meet some good people. At grass roots level you'll probably help some people, but if that's your intention then NGO's are your best bet and that's something else altogether.