Haha, I love the Feynman Lectures, but the only reason I can make head or tails of it is because I'm using it to brush up on subjects I've already studied before
I would describe the Feynman Lectures as highly understandable in the same way that I would describe the Art of Electronics as highly understandable: while you will definitely benefit from prior exposure, these are the books I would choose if I could only read only one book in their respective fields. While they take more effort to understand, I'm pretty sure I could have read them coming out of high school and would have had a better understanding of the subject matter than I would have gained from the eclectic mix of textbooks I did learn from.