Depends, do you plan on keeping a lot of SPS and clams? Or mostly softies?
Another way to automate calcium dosing for low-demand systems is to use kalk for your top-off water or run your top-off water through a kalk reactor. This will only work for lower capacity systems, but it should be just as rock steady as a calcium reactor if you use accurate valves to mix RODI back into the Kalk after the water, and it's maintenance free and much cheaper than a Calcium reactor. That said, it will only work if you really have a low Ca useage, and/or high evaporation rate.
Nate