I think a good PPI can catch a lot of potential issues, and obvious problems most people would never notice. It won't do much good with regards to catching IMS failure, but if someone claims the car they are selling has an upgraded IMS a PPI can identify certain types of upgrades. Service documents in my opinion are as important as a good PPI. If you know what to look for you can learn a lot about a car. The flaw with just documents is that something could be missing, like the repair bill from a fender bender.
I just bought a (small) sailboat, can confirm, tearing up hundred dollar bills.
