I find it works OK with about an inch of bark/coconut fiber. The original reason I got the seedling heat mat is because it's big and was only $30. I needed to heat one end of several shoebox tubs. It's just the right amount of heat for that. The reptile mats are too hot and would cost me nearly $25 for the smallest size.

Yes, the one I have is big. Big enough to heat the entire bottom of a 20 gallon long but like I said it doesn't get very hot. Mainly I'm using it like that to provide a little heat at night for babies. It's on all the time but during the day, I have a 60 watt incandescent bulb over one end, in addition to the heat mat. That brings the surface of the substrate up to around 31 C. The remainder of the bottom, on top of the substrate stays around 23 C.

So yeah, I'm using it as a supplement rather than an exclusive source of heat. It's just right for this application and an inch of substrate and like I said, for tubs sitting directly on it with paper towel substrate, the temperature is just right without getting too hot.