The Feynman Lectures are great at giving you an intuitive understanding, but is no substitute for the regular curriculum. You don't find many people who read only the Feynman Lectures who can then go on to solve physics problems well. You do find many who read the regular textbooks and who can.
You have to bear in mind that the lectures in The Feynman Lectures on Physics were only one third of an introductory physics course, the other parts being recitation sections (in which homework problems, quizzes and tests were given and discussed), and labs. Lecture attendance was optional - many people prefer reading to listening - but the recitation sections and labs were mandatory, because they were considered much more important. Nobody learns physics from just reading lectures.
