The Red Book holds a central place in Jung’s works despite not belonging to the Collected Works. It is also an unusual book, the same way Thus Spoke Zarathustra is a special book.
My description of The Red Book is that it has a dreamlike quality with very lush prose and strong imagery. Reading it might take you for a journey or completely throw you off. This is why I suggest not to read The Red Book unless you feel that the book is calling you.
Note: as of late 2020, The Black Books have been published and I have gone through them. There is no reason to read The Black Books over The Red Book because they are essentially made of the same material (at the exception of half of Vol. 6 and Vol. 7). Compared to The Red Book, The Black Books are composed of only personal entries with no additional commentary, making the reading experience impenetrable even for seasoned amateurs. My recommendation then is to let The Black Books aside until you have read and understood the majority, if not the entirety, of the Collected Works.