It works well. Although a bit inconsistent, you never get rapid fluctuations. Day to day my alk is always within the margin of error of my test kit. Week to week, + or - .3 dkh at the most. The ph buffering is consistently 8.3. Prior to kalk, it was 7.9. That’s another bonus.To be upfront. I am not presently nor have I ever used kalk. However I am prepping to do so and have done a considerable amount of reading. But since I am not speaking thru experience I would suggest you listen to others as well.
If you can, look into a kalk stirrer. They are a clean long lasting metered devices (but not cheap)
Kalk in the ATO has a lot of issues... it does work. It is simple probably the cheapest solution. Down side is. Possible pump clogging and messy. But the single biggest issue is you are counting on evaporation and evaporation is not consistent. So the more you evaporate the more kalk goes in and obviously less on days that evaporation is less. Ask anyone. The greatest thing you can do for a reef tank is consistency. You are better off having a parameter constantly a little off than chasing it up and down trying to perfect it. In my opinion, kalk in ATO. Is by nature a built in inconsistency.
Now with all that said. It does work. And chasing kalk is not as bad as chasing Alk and PH. So take all this negativity with a grain of salt