The grip points are bevelled and therefore do not maintain adequate grip on the floor. As the chair moves around so does the mat. Also the plastic used could be a bit thicker. Returned this mat and bought a different brand which works better.
i would have been willing to pay more money for a thicker mat. I know this one will last only for a few years. I suggest a thicker mat that will last a minimum of five (5) years with normal wear & tear.