I feel like I must enter the debate, since I graduated from the Graduate Institute recently, and I am well aware of the reputation of both GSD and the Graduate Institute.
In a nutshell, the Institute has a pretty good reputation worldwide - although the school's size limits the reach of its sway - and it has a stellar faculty. It's strongest departments are international law and political science. The premises are so so at the moment, but this should change when the new campus opens in 2012. A lot of the school's students come from the top universities of the world. I think that this may be the best indication of the perception people have of the school.
GSD is, as far as I'm concerned, not much of a uni at the moment. Don't get me wrong, it probably has potential, but it is perceived in Geneva as a school that admits rich kids that cannot get into a good program. I think the only positive side of going there is the contacts you can make. But hey, you will also make contacts at the Institute or any other decent uni, and you will also get an education that will not cause people to raise their eyebrow. In short, I think that portraying it as a good uni is a bit misleading, and this should be known because grad school is a big investment.
And to reply to carcass2, I doubt that GSD has produced ambassadors, and this is not because of what I think of it. Rather, this seems pretty unlikely because the school has only opened its doors some 8 years ago, meaning first graduates entered the job market 6 years ago. It takes a bit more time to rise to the position of ambassador.