a) It took me about 30 minutes to fully review this site. I went in and out of each links, spending more or less time on a topic that interested me or less, or I spent more time on topics that I didn't know what they were. For example, the section titled "The Planimeter" was about a mathematical tool that I hadn't previously heard of, so I spent more time there than I had for "Chinese Handcuffs".
b)I think that the site does a good job at explaining concepts. In fact that is likely it's strongest point, which is that it accurately (at least to the best of my knowledge) conveys information in such a logical manner. Concerning how educationally significant they are, I think the writer of the site sort of takes away from that by explaining in the very first paragraph that he doesn't feel the need to relate these mathematical concepts to students every day lives. True, that not everything can be related. But it takes away from any possible building interest to read that as a student. So yes, the materials are educationally significant, in the way that all things can be significant, but the author is unable to show students why/how this is important to them.
c)Certainly if a student were doing a report on any of these topics, or if a teacher would like to give students additional resources to review on their own, this would be a good site for that. It's very well written and concepts are explained very clearly, in my opinion. I especially think that the way concepts and topics are clearly organized, which always makes for better teaching/learning. Too often do educational website become too muddled and confusing with poor page design, which ultimately intimidates students.
d) The material is easy to use, and the files or Geometer's Sketchpad aren't extremely large files, so the typical student's laptop should be able to download it with ease. Like said before, concepts are explained well and topics are organized, so it's impossible to get lost or confused amongst the materials.