If you haven't read through First Aid on your own, it's best to do it with some assistance, I find it helps getting through it quicker (if you are a slow reader like me) and doesn't allow your mind to wander (at least on the first go). But afterwards, I would only use it in areas where you are having difficulty with. For me physiology was the absolute bane of my existence during Med 1, so I completely reviewed all of my physiology with DIT, Costanzo, First Aid and anything I could get my hands on.
What it's exceptional for:
» Behavioral Sciences
» Bio-statistics
» Medical Ethics/Medicine in Society
» The Early Foundations
These portions of the course offered the most new material I had seen before taking my Step. For me this was all new material and I took my time, watched it at the very least 5x and annotated my FA when they said something I had seen in a question or found semi-relevant.
I would recommend going through the whole course at least once in your light preparation stage, for me that was back in February 2015. The course itself shouldn't take more than 2.5 weeks and the rest of the time (1.5 weeks), I would spend reviewing FA, doing questions and gearing up for NBME at the end of the month.