I learned my own lesson about a week ago... Bought a Certified Pre-Owned Mercedes E500. Car is 2 years old and has 10k miles on it.
I saw the car in its lease-return state and it was filthy inside and out (while I never detailed a lease return before handing it in, I at least never left...
I broke open a brand new bottle of Wolfgang TSR (pre-3.0) that I bought in December last weekend to try on my '99 MB 320CLK -- I was very disappointed with how it worked. It didn't appear to do much to the hard clearcoat on the car. I figured it would take too many passes to correct the paint...