I just took and passed the PST test.
I noticed the following things:
- I did a lot of preparation. I worked through the McK practice tests a couple of times. I did some of the external tests provided by a few websites (they were on a different level most of the times - best preparation was the McK tests). Total maybe took 10 practice tests (2 times the 3 original ones, and whatever I could find).
- Once you know the type of quetions they are asking, it is pretty straightforward.
- I found a lot of people that showed up on the day were less prepared: They had only done 1 test, heard about the test a week before, thought it would be a 'either you have it or not' situation.
Therefore, my personal view is that the PST is a really good way for McKinsey to weed out applications that just apply for fun and games, and don't really want to put effort in. Those people may be capable to do the job, but doing the test without preparation is hard (I had to let go of some GMAT logic as I was just answering a wrong question...).
If you prepare with the materials available and speed up your maths, you should be good.