The book is relative short, but all Feynman writings are excellent.  Dispite holding a degree in Physics, I did not find the "pieces" all that easy, but very worth-while reading.  The lectures increased in difficulty, but all helped me understand the subjects better.  (I'm only bitter about not being aware of Feynman and his writings / lectures when I was an undergrad in college.)  I expect to re-read this little volume multiple times, because dispite it's claim to be "easy", the material is very rich, and mostly Feynman leads you from the known to the unknown.  I certainly recommend this book to all students of physics, from high school students to graduate students.  Other science and medical students will probably benefit from it also.  Interested layman may find it worth attempting.