Solar Rings

Are the solar heating rings that various places sell actually useful for heating water or no?

based on numerous comments by people that have bought them and reported back on this and other forums...in a word, no.
It seems that a slight wind blows them all over the yard.

Also, solar blankets, rings, Heatsavr liquid, solar fish and pills, etc. do not actually heat the water. They help prevent heat loss from evaporation to a greater or lesser degree. Evaporative heat loss usually happens overnight so to maintain maximum heat in your pool you want to cover it at sundown and remove the cover during the day so the sun can heat the water.

Solar blankets seem to be the most effective of all of the above, Heatsavr based products (solar fish and pills) the least. There are many types of solar blankets at various price points but general consensus is that 12 mil seems to have the best price/performance ratio (how long it lasts vs how much it costs) and that the color (clear, black, silver, blue, silver/blue) does not seem to have much effect on how well it works.

I have heard that in Scandinavian countries they keep the heat in hot tubs by covering the surface with ping pong balls. I always wanted to try that if I could ever find a cheap source for enough balls to cover my pool and spa. They do not have to be removed when you use the pool or spa but if you wanted to a leaf net or leaf rake would remove them fairly quickly.

I had the solar rings several years ago. I found them to be a waste of time and money.
I actually would have to retrieve them from my neighbor's yard when it was windy or a storm would pass.
The other issue I found it almost impossible to get them to lie flat on my pool (which would explain issue above).
I was able to return mine for a refund and go back to the trusty solar cover.