If you have a high calcium/alk demand system, which pretty much means you have lots of SPS, then the reactor supplements the calcium and alk that's consumed by the corals.Some people love them because they eliminate the need for dosing, but lots of others tried them and went back to dosing calcium and alk seperately. They do take some adjusting. You need a controller to handle turning the co2 off/on and you need to do alot of testing to dial in the reactor. Too little co2 and not enough media is dissolving to keep up with demand, or too much co2 and your alk shoots up and kills your corals.
I perfer the manual DIY 2 part dosing.