Glassware – the use of actual lab equipment – is probably the #1 biggest difference between these Checkers and the kits most folks are used to. And the source of most of the complaints. The do require attention to some different details than someone might be used to.
Spotless is no exaggeration. I even recommend folks buy Hanna's cleaner cloths (or something like them) if they are available....price is reasonable, you can dedicate one just for this purpose and no other, and they are MUCH better than about any other solution.
(That eliminates any doubt, but any good lint-free cloth will work if you know what you're picking – natural fiber or these microfiber options.)