I'm not sure where Spensar is getting his information. The insulating part is correct, but the heating part is not.

Solar covers can be VERY effective at heating your pool. They aren't nearly as effective as solar panels but they do work. It's just a matter of how much heat you need.

There's some debate as to whether opaque covers or clear covers work better. Opaque covers work by heating up and transferring the heat to the water by conduction. This does work and the water 4 to 6" below the cover will be quite warm, even hot. This then has to be distributed by the filter.

Clear covers work by acting as a greenhouse. The sunlight penetrates the cover, heats the water but the heat is trapped, rather than dissapating up into the air. This, too can be very effective.

Clear covers seem to work better when they are thicker providing more insulation.

My personal opinion is that thick, clear covers work best, followed by other clear covers, which are pretty much the same as opaque covers. Blue transparent covers are the worst. They don't pass enough light to heat effectively, but they don't heat up enough themselves to conduct heat to the water.

No matter the warranty all covers seem to go after about 3 years. They stretch (mine is already 2' longer than it was), then the bubbles start to pop. When they are rolled, it's best to keep them covered with a white opaque cover. Otherwise the sun bakes them and the inner layers get like an oven and start breaking down.

One thing I don't understand is why opaque covers are blue on top and black on bottom. I would think that black on top would absorb far more heat to be transferred.

Anyway, I've had experience with all three types and go for the thicker, clear covers now despite the expense.

A good roller is also a must. Those plastic rollers do not hold up, but I'm much happier with a metal-framed roller.