well, like I said, I like to let them have a bit cooler of a cool side than most people do. Don't get me wrong, radiant heat panels are great, and if I ever have arboreal snakes, that's all I would use. But I think an animal has alot more control over thermoregulation when they can lay in direct contact with whatever temperature they desire. just like if you lay a pot of water on your electric stove. It heats up and boils, regardless of air temp. Here it depends on the species though. I find my retics are quite happy to take advantage of that cool side down in the high 70's. This is definately a good topic for another thread! Didn't mean to hijack this one... lol, but yeah, I've tried em all and I will stick with belly heat for my terrestrials, which just adds one more reason to a long list of reasons (for me anyway) not to use "substrate".
CinderellaBoy beat me to the post... lol
As far as natural heat sources... well, I gave up on the natural thing a long time ago. And like I said, if we try to relate these things to ourselves, I for one definately prefer my furnace and air conditioner over a fire in a cave or waving a leaf at my face....
Again, not knocking radiant heat panels by any means. Just been there, done that, and I see better results as far as growth, reproduction, and stress levels with belly heat in my animals.