I've read lots of reviews of the Thermarests. Quite a few people rave about them, saying that the comfort margin between cot and mat is as great as the margin between mat and ground. When there's a cold spot under the torso because body heat is lost, people put clothing under their sleeping bag, or some foam, or a 90 gram reflective polyester sheet sold by Thermarest. This is billed as converting the cot from 1 season to 3.
Some reviews say the cot sinks in the middle too much and their body is resting on the ground. It seems to be a very YMMV product. I've had some decent nights' sleep on an army camp bed from the '50s with 3 transverse bars, so I have quite high expectations of the Thermarest, which has 6. And the Thermarest bars can be fitted in different configurations to alter the amount and the location of the support. I certainly wouldn't assume it's no more comfortable than the alternatives without trying it.