First, a disclaimer: the online exercises of this course are extremely thin, so your score in this class is neither necessary or sufficient to gain mastery of the material. It helps if you think of this course as supplementary material for the book (An Introduction to Statistical Learning by James, Witten, Hastie, Tibshirani). In this light, the course becomes an exceptional gem, because the book is really incredibly good. My recommendation is to take the time to read the book cover to cover, trying many of the excellent exercises in it. Then, as a recap or a refresher, go through this online course. The lectures highlight the most important parts of each chapter and are beautifully paced and presented. You will find that they are a perfect complement to the book and many concepts will become clearer and more concretely established in your mind. However, if you try to take this as a stand-alone course, you will be disappointed and likely not learn or retain very much.