I think it depends on what you are using it for. Will it be your only means of calcium / buffer? I use mine in addition to a large calcium reactor, mostly to counter the pH drop I get as a result of having to use so much CO2. There's no way kalk alone could keep up with the demands of my system which is loaded with SPS, about 260 gallons.
I just recently got a Geo and so far it has been great. I don't see anything unsafe about it. I am running it with an AquaMedic reef doser on a timer which runs for 15 minutes, 8 times per day. The stirring pump on the Geo runs for 30 minutes, 4 times per day.
I had an MRC prior to this, with the magnetic stir bar, and the unit was too small to keep up with my system. I think the Geo is the best one I have seen from a design and quality standpoint.