I don't really like it. Since Step 1 isn't based on this format, it doesn't seem all that useful so far. (In case you don't know, you get a case description and then 5-7 questions are asked about it in succession.) The questions in the vignettes are usually dependant on getting the correct diagnosis as the beginning. That means if you get the diagnosis wrong, you'll likely answer all the questions incorrectly. I don't see how that's supposed to help you learn. It actually makes me really frustrated because my problem isn't knowing things about certain diseases, it's getting the diagnosis.
I personally prefer the regular Qbank. It does what I want it to do. I only have IV Qbank because I also wanted to buy a full-length in-center simulated test in addition to regular Qbank and it was cheaper to get all three as the package.