I used one on my 65g for about a year. The biggest plus to it is its simplicity, but it is hard to tell if you are getting a fully saturated kalk solution, since there isn't a lot of mixing.
I finally broke down and bought a kalk reactor. So now I know I am getting a lot more mixing. I was hoping to be able to decrease the amount of 2 part that I'm dosing by getting a fully saturated kalk solution. Interestingly enough, I am still dosing the same amount of Ca/Alk supplements, which leads me to believe the Tunze version must have been doing pretty good.