The only reason I haven't purchased the calcium checker is that the testing process seems to be quite involved. Also the refills for the checker cost almost as much as a Salifert Calcium test kit. Not worth it in my opinion. I get satisfactory results from the Salifert kit.
Now when they come out with the nitrate checker, I will probably get one of those, and possibly the mag checker. Have to see what testing process and refill kits are like.