It really depends on the vendor. I have bought PLR that even with great writing skills, I cannot make into anything useful. Then there is PLR from sellers who I barely have to touch if I don’t want to. Even with the latter type, I will tend to lightly rewrite just to add my own “flavor” (or flavour for those who use British English 😉 ).
The one thing that I have heard often is that you should change the graphics and the name of it. This makes sense on one hand, but on another, it can be disheartening to find out that the PLR you just bought is almost the same, or the same, as something you already had. There is one seller who I have seen who identifies the PLR she has gotten for resale from other vendors, and advises would-be buyers to check to make sure they don’t already have it.
As a writer, though, even though I have a lot of PLR, I often find it just as easy (or easier) to write from scratch. It’s weird, but true.