When I first started out, I had my baby in a 20g long tank with only the background and one UTH. I learned as I went and got the thermostats and second UTH and the temps improved...but once I added the insulation (cork first, and then the foam board) I noticed that I didn't need the lamps for ambient heat, and my thermostats weren't coming on nearly as often. That's mostly what you notice - stabilization of the temps.

You don't notice it much in warm weather, but in the winter it really makes the difference!