When considering a solar cover type device such as these, I like to reccomend that folks read the US department of energy's writeup on solar covers here: http://energy.gov/energysaver/articl...ng-pool-covers

It explains how and why solar covers work. Primarily by preventing evaporation from water that is exposed to air. One of the problems with the rings is that they don't cover a lot of the water due to their shape. The other is as mas985 said; they blow away if there's any wind.

I decided to not use a solar cover myself this year, although I could potentially get an extra month or two longer swim season here in FL. I find that I'm just too lazy to remove the thing and replace it after and the result is I don't swim anyway.