I think my carpet is a medium pile and I have been using this chair mat for a couple of weeks and it has some pretty serious dimples where the wheels of the chair sit. I make sure the chair is not in exactly the same place on the mat as I believe that would make the dimples worse. I will keep an eye on it and update in the future.
The product description said this product would be fine with pile carpets. It is not stiff enough - so the chair creates dents in the mat. They are not permanent but with a rolling chair you are constantly stuck in the valleys that it creates.