Solar pool collectors are usually some of the simplest and cheapest collectors there are. But they will also give you some of the biggest bang for your buck. Heating pools is not cheap by any means except by using solar energy.
Pools that do not need to be heated year round can use a cheaper rubber or semi-rigid plastic collector. These collectors can extend the length of the swimming season for your pool but generally do not allow for swimming year round.
If you need or want to heat your pool year round then you will need glazed enclosed collectors or evacuated tube collectors. These produce higher water temperatures and therefore higher pool temperatures. But of course they are also more expensive to buy and install.
Solar pool heating systems will cost more upfront to install than the regular gas or electric heating systems but you will easily get your money back in a very short time. Once installed and working the solar pool systems will cost almost nothing to operate versus thousands of dollars yearly for conventional pool heating systems.
And solar heated pool systems can easily be built and installed by the do-it-your-selfer. The systems do not need to be installed on a roof or high structure and can be mounted beside the pool or close by.