Yes, I have. I have personally had it read well below the correct dosage when the other two read correctly. You have to remember that Cupramine is a unique form of copper (complexed to an amine ion), so kits that are designed for ionic or chelated copper will not give accurate results. But, if you feel you are getting accurate results, so be it.
In general, API kits are more for gross readings than precision IMO. But, that's just me. I'm not "hating" on the product; I just won't use a product that could potentially put my livestock at risk.