I'll paste what I posted elsewhere in response to the same question:
I did make it to the open house. The campus is beautiful, and for someone who went to bland, non-descript commuter campuses in San Diego, I was definitely impressed by the architecture and the facilities. Duke University has money and isn't shy about spending it. Just touring the student gym facilities and libraries can sell you on attending. The staff and student ambassadors were very friendly and really went out of their way to make sure that you talked with people studying in your intended field. As someone who is looking into Environmental Policy, I was practically dragged around during social mixers by student ambassadors to meet and greet with current MPP/MEM and Enviro Concentration students. The "breakout" sessions were also very helpful. Unlike GW's open house, Sanford School held breakout sessions for each major concentration, where you got to hear from professors and current students. There was also a very informative "student life" panel. I can't emphasize enough how friendly and helpful everyone was. There is a very collaborative environment there - exact opposite of more cut-throat programs out there. In fact, group/teamwork is a basic element of most classes.
All in all, Duke and Sanford School were awesome. Any negatives I felt were either about the location (Durham) or based on my mood (exhausted from my West Coast flight and the all day open house). One of my few reservations has been about Durham itself, which on first glance is not an appealing place to live. Following that open house with my GW visit didn't help, since DC is such a dynamic and energetic city. Bad comparison. That said, I had very little time to explore Durham, and no rental car, so my first impression isn't very fair. And heck, the program is only two years long, and I think most of us could "tough it out" for two years in exchange for an awesome degree. If you have any specific questions, please let me know.