Most RO units have a DI stage, so how could a pure DI unit be better then RO + DI? DI only removes the chemical elements that are attracted to the resin. It does not remove any solid particles and any impurities (Alot of municiple water supplies have many impurities, like oil, in extremely minute amounts). It works on the same concept as a water softner. There is no mechanical filtration, just chemical, which is not 100%.