Never used them, but I have used a block of wood with a degree of success. I would definitely measure your jack to make sure they're the right size before ordering. Also make sure the extension is not so big that you can't get it under the jack point when the car is on the ground.
With hondas, I've had more trouble finding a floor jack that would go LOW enough to get under the jack point, rather than finding one that would go high enough.