Yeah, the BSS books will kick your ass -- hard! I just finished the first book in the series, General Principles in the Basic Sciences. My average score correct was 70%. As the above poster said, the questions are not in the current USMLE format. That being said, I liked the challenge of the questions. The writers made the questions super-duper hard. For example, for one question, they showed the citric acid cycle and instead of putting in the real names of the compounds in the cycle, they labeled them A,B,C,D... and asked a bunch of questions. The explanations are also very well-written. However, there are some really stupid questions that you have to sift through. For example, some questions want you to calculate the crude death rate or use terms that I've never heard of, such as "nephelometry."??
From what I know, people either really love the series or hate them. There is no middle ground 😱