I have a small 12" square one made by Olfa. I use it for squaring up small blocks (under 12"). It's one of the ones where the mat has a rigid plastic/acrylic back with a protruding circle that fits into a plastic base. It's very flat, no lip and no moving parts to wear out. The mat itself has lasted me for a good long time, though there are lots of surface scratches from years of quilting. They don't seem to affect the accuracy of my cuts when I'm squaring up blocks.
Rob