The ADA knock offs work fine. Just ignore the instructions that come with almost every drop checker since they tell you to put tank water in them. If you have a standard pH test kit (bromothymol blue) you can toss the reagent they give you and use the reagent you already have. The reagent with the cheap drop checkers is probably fine, though.
Originally drop checkers were used to show you the pH of the tank water that is why the directions say to put tank water in them. I am pretty sure Hoppy was the one who figured out you could put 4 dKH water in the drop checker with the pH reagent and that it would turn green (neutral pH) when the CO2 concentration was around 30ppm.