Hannah Instruments is a professional world renowned company, focusing on testing various elements, so the best thing is you know that their equipment and tests, reagents, etc will be accurate - that's why I would love to have them.
Also, what many people forget is that our individual ability to "see" certain colours varies from person to person, is influenced by the light you are viewing in, and the hanna checkers remove this variability.
I think the best thing about these is you don't have to try to guess what exact color you are looking at, or mixing trying to mix half amount of chemicals to extend the number of uses and not getting the exact amount need.