I ordered a chair mat and when it was delivered it was very dirty and had several big bumps in it. It was not flat and took two days with books piled on it to lay flat again. I was expecting to be able to use it right away like the last time I ordered one.
If your chair stays in one position for a while you need an extra 'shove' to get the wheels moving again. I would still probably recommend it with that caveat in mind