I grabbed one last week. Used it a few times. I will say that it seems quirky at first, then seems to settle in to proper tests. One note is...make SURE the vial is cleaned out with RO/DI water after every use and make ABSOLUTELY sure that the RO/DI water you use for your test is absolutely 0 TDS. If it's off, like you rinsed out your container with tap water then filled it with RO/DI, your test will be off. However, with a clean vial, perfect RO/DI water, and following the steps on that video, you will get very consistent results from the checker. Oddly the results were different from my salifert test (hanna showed higher Ca), but the salifert is sort of ambiguous as to when the solution changes colour, so it may just be me.
Again, contamination of your testing materials can give you a absolutely wonky result. Remember, you're only using .1ml (6 drops) of your tank water for the whole test. 6 drops.
The reagent dissolves a lot faster than the phosphate one though, so that's something. Having to have RO/DI water for the test is a bit strange, even if you buy it from the LFS, it could be off or contaminated, so a handheld TDS meter is almost mandatory for this test.