I've taken the ACS General Chemistry and ACS Organic Chemistry exam. Honestly, it is pretty thorough and the only way to study for it is to study the topics you are weak in (rather than study EVERY topic since we are human and only have so much time on our hands!). Getting an official ACS study book is a great way to do this. The problems in the ACS book are similiar to the ones you will see on the exam and therefore it is good practice.
I never found the exam to be extremely difficult. If you have understood everything you have been doing all year, with a little review you should do decent. There will be questions that you will not know.. could be because your professor never covered that material or simply because you forgot. Try and do all the problems you know first and then go back and look at the ones you skipped and just take an educated guess. Another thing to note is that these exams are graded on a curve. For instance, my grade on the ACS General Chemistry was 51/70 (they are 70 m.c questions) which translated to an A- on the scale. So, all in all, it's doable.