This book has allowed me to verify some of the concepts I thought were true, based on other books I have read but did not totally understand.  Feynman does not presume that the reader can mentally leap from one mathematical concept to another without a conceptual bridge which he usually provides.  His examples around how the longest elapsed time is the shortest spacetime distance are excellent.  His explanations of time dilation due to speed and gravity are very clear.  I really like this book.  The subjects in this book can be understood by anyone who has some physics and math background but it is not going to work for someone who has no background or has no intention of taking some time to ponder and study.