This is with the solar cover on. I would say that in Arizona they work quite well, and maybe are essential, but in the Midwest they likely could be more trouble than they are worth. I do need to get my roller system done for that cover soon, it's a pain to take it off and put it on at present. I am working on getting the sun shield for the equipment done, and then the cover, and probably a Circupool salt cell is next.
But I would guess if I didn't have that cover on, I would be about 5F less now and we would not be swimming. According to my weather station the average temp for the year is about 4F less than the water temp, so that's a rough estimate of what it is doing. I also assume I have kept a lot of water from evaporating out.. I would guess almost none over the winter.
It also acts as a winter cover in the warm (but not hot like in Florida) climate. We get to a low in the mid-20's in January, but that typically happens 2-3 times and lasts less than 4 hours. We HAVE seen snow here, but that usually only lasts a day before it completely melts. So really it should be covered when we are not using it but we really don't need anything more.