I had it about a month ago as my final in ochem. The format was a lot different than our regular tests being that ACS is multiple choice. I started studying 2 weeks before with the ACS book. Basically, I would get to a section in the study book, and before doing the problems, I would review that section in my text book and make sure I understand all the mechanisms and reactions. I would then work the problems in that section and record any tricks or nuances on a sheet of paper. The day before the final i went over all the study book using the nuances for the tricky questions I might have had problems with. You only need a 50 out of 70 to be in the 90th percentile, this is what I had to get to earn in the A in the course, and thankfully, my technique worked above.
P.S. Although I do not like guessing, it can help on this exam (there is no penalty), for instance: I found that on the test if I had absolutely no idea, or was trying to narrow it down, ussually two of the answers would have something in common that set them apart from the others, for example being conjugated alkenes, for me in 90% of cases, the right answer was one of the two. Again, I'm not advocating guessing, but on some of the questions you are unsure about, you can definitely use some logic. Hope this helps