My school had the same deal. I didn't have it for my first rotation, which was OB, but I used it throughout the year to study for the Shelf exams in my other rotations and for Step 2. I always did pretty well on my Shelf exams (above the class average) and did really well on Step 2.
I was afraid that I would run out of questions or have the answers to the Q-Bank questions memorized prior to taking Step 2 CK. This was not the case. I had forgotten the answers to most of the questions from my earlier rotations by the time I started studying for Step 2 and Q-Bank was updated a couple of times this past year so I always had new questions to study from. Using Kaplan Q-Bank throughout the year was definitely beneficial to me.