I have medium pile carpet and measured depth per the instructions. My office chair wheels sink into the plastic and it's difficult to roll my chair out of them. I might try the plywood under the mat trick but would rather find a thicker more rigid chair mat that is sufficient.
I'm pleased with the Medium Pile Carpet and Hard Floor Chair Mat I purchased. I use it as a cover on a large throw rug and it doesn't shift or move on the rug. My office chair rolls smoothly on the mat.
This mat fits perfect in desk area without that annoying flap. However it does move around a lot. Even when it says its for low carpet. It doesn't have the spikes that holds it in place like the others and priced WAY too HIGH. But its wide that's why I like it. Haven't cracked yet so its pretty heavy duty.
The chair mat came flat not boxed which was an initial concern. However after cleaning it I placed in on the carpet and it was completely flat no curls at all. While a tad soft so the chair leaves an impression I am able to move around easily. Perfect mat for my needs.