Not at all. I passed, with a some breathing room, but I was squarely below the national average.
I did about half a year of PREP. Its explanations were extremely comprehensive, which is both a good and a bad thing--you learn a lot, but it take a while to get through. In a perfect world, I (and your cousin) could have been more religious about studying throughout residency in which case PREP is great. I found that I didn't have time to go through them or a textbook while I was scrambling during a busy fellowship, so the more direct question form of the Medstudy questions was better for me as I could just get through them faster. As I said above, I also found the format of the questions closer to what was on the real test.