I owe it to my CAPM friends who told me to take the diagnostic exam before the CPSM. At that time the CPSM diagnostic test was not ready so Bruce Caldwell (then CPM now also CPSM ) was so kind as to drop off his old copy of the CPM diagnostic test. Thanks Bruce !
Before I got hold of the CPM diagnostic I was really going through the 3 books - reading a section and putting out a blog post in this blog (at least this blog was more regular). Once I took the CPM diagnostic - I immediately got to know exam topics where I needed to strengthen my studies.
The only problem was that the CPM syllabus and approach was limited and the CPSM was more like an MBA or GRE (Advanced)/GMAT type of test in the broader supply management domain. In any case, the CPM diagnostic kit was invaluable.
Today, even if you have spent years in supply management and have read all the material I would strongly advise you to take the CPSM diagnostic test which is now available. Take the diagnostic test exactly as per the time allowed so that you get a real feel of the exam and take it ahead of the real thing so that you have time to review stuff that you need to strengthen. Check the ISM Website for more details here.