Firstly, I came to this book with very little knowledge of mathematics other than the high school math I finished around eight years ago. However, I have a general interest in some of the abstract mathematical ideas, and I was also keen to expand on my knowledge for the master's degree I'm currently doing in which a little bit of math knowledge would be beneficial (although not a requirement). Given this, I didn't always find the book easy to understand, especially pages ~200 to ~300 (calculus, vectors, matrices, abstract algebra, and complex numbers), but I did find it pleasurable and quite addictive, nevertheless.
I really liked the format of the book: each of the 200 concepts is described on two pages - the first page giving a written description and the second page showing some relevant graphic or picture. There are a few slight deviations from this format where the written description spans two pages, but the general approach of the book is to describe the main properties of each concept in around 200 words. This makes the book very easy to read in short bursts. The converse of this approach is that each concept is dealt with pretty superficially, but for me personally this was not a bad thing.
Although superficial, the book didn't feel dumbed down - it doesn't shy away from using formulae, for example - and I imagine the more advanced reader might also find the book useful for recapping and seeing the connections between disparate mathematical ideas.
Although I can't really comment on the quality of the mathematics, I felt the quality of the book deserved a 5-star rating. I also think the price is very reasonable too. The only fault I can find is that some of it was a bit hard to understand, but this is probably more my weakness than the book's. Given that I enjoyed this one, I think I'll probably also buy the other two titles in the range, Science in Seconds and Big Ideas in Brief.