Not yet, but one day I will. Am in the process of building one/convincing myself to buy one.
Basically, what they do is they are filled with aragonite, which is broken down by water which is cycled through it that has a lower pH due to the injection of CO2 into it. As the water passes through the aragonite medium again and again, it breaks down/dissolves the aragonite, releasing calcium and other stored trace elements into the water. The pH gets built back up by the process, as aragonite will buffer the water, and the effluent goes back into your tank, adding important trace elements and calcium, and keeping alk higher. The pH of the effluent is often lower than what is desired, so sometimes a second chamber is added as a final pass through which will buffer the water a bit more. It introduces this water at a slow rate to the tank, usually a drip or a tiny tiny stream which will not affect the pH of your tank.