I haven't taken Step I yet, but I have taken practice tests from various sources (NMBE released step 1 questions, Q-book, NMS, and BSS) and the BRS Path and Phys questions always turn out to be quite a bit easier than any of these question sources (90-95% performance average for BRS versus 70-80% for the others). One complicating factor, of course, is that questions from the other sources come at you from all directions (all subjects), but the nature of the questions still differ.
If you look at the format of the BRS questions, many of them aren't even in boards format (paucity of vignettes, EXCEPT questions, etc). They are meant to test your recall and understanding of the material presented in the books but aren't apparently meant so much to simulate Step 1.
These are released Step 1 questions from the 2000 examination. If you go through you'll find that some of the questions are no-brainers but a few are a bit on the more difficult side. As a first year, though, you'll probably draw blanks on path, pharm, and micro questions, but you can still get a feel for what the questions are like.