A thin chair mat on the carpet in front of my computer let the wheels on my chair dig in and keep me from rolling. Staples solved my problem with a new thicker mat. They even delivered it.
After reading these reviews I knew there was a possibility that my chair's wheels would sink into this mat somewhat and that was okay with me (I hadn't found an alternative that I liked). So even though the wheels have made indentations in this mat the berber carpet underneath it no longer has any. Plus I can roll my chair around much more easily on this mat than I could on the carpet. I am pleased.