In my opinion, this site is a GREAT way to practice for the GMAT, but beware. People tend to post many questions that are MUCH more difficult than the actual GMAT. I also remember facing a particularly difficult hexagon question on a MGMAT practice test, but these questions were way more difficult than anything I saw on the GMAT.
Now, I have only taken the GMAT once so I'm not saying that I know the entire database of questions, but I would say it is very unlikely that you will get any such question. Obviously understand the basic dimensions of most shapes, but do not kill yourself trying to understand every little detail. The GMAT, the math section in particular, is more a test of the ability to detect trick questions and to work under strict time conditions rather than understanding difficult math.
Just my opinion.
By the way, thanks for your encouragement with Duke, I would love to go there, especially since it is relatively close to Hilton Head Island (where I grew up).