My chair cause divits in the mat after only sitting in the chair for 10 minutes. They have not come out. Everytime I sit down in the chair it caused more divits then you can't move your chair. I got frustrated and threw the thing out and purchased another one.
Works ok. If you sit in one place for a long period of time the wheels on the chair sort of sink in making it a little difficult to move. At least it isn't cracking like the one I had to replace.