I've used them before. They are an ion-exchange resin filter. The # of gallons you get from each cartridge varies by how bad your water is. The worse off the water is, the shorter the cartridge life. The cartridge changes color as it nears exhaustion.
They are cost effective for small tanks only due the cost of the cartridge. For larger tanks, full blown RO
units are cheaper in the long haul.
Best of luck,