This is awesome. Mathematics struggles a lot from bad presentation, but this does an excellent job of taking well-known and interesting problems and explaining them (and their solution). Problem solving is a lot more fun than simply reading mathematical truths.

For the areas of math that I know well, the content is exactly what I would expect -- i.e. it's exactly what I would say to my non-math friends if they asked me what I was doing in, for example, my graph theory class.