OK, here are the numbers: Yesterday's high here in Battle Creek, according to weather.com, was 76 degrees and the low was 50 (probably early yesterday morning after the rain). Right now (7:30 a.m.) it's 53 degrees. My solar panels yesterday managed to raise the pool temperature five degrees, from 68 to 73. That is typical for such a low starting temp. After checking my temperature around 6:30 last evening, after the pump shut off for the day, it was right at 73. This morning, with the outside temp at 53, the pool is still at 70. I find that to be acceptable, not knowing what it would be without the Cover Free, and not knowing what it would be with a thermal covering.
I understand what some others are saying about wind. That certainly would be a major factor, but for me it's rarely one. My pool is well protected from the wind and almost always calm. Of course, you couldn't run the pump at all overnight either.