In the United States metric units, authorised by Congress in 1866, are widely used in science, military, and partially in industry, but customary units predominate in household use. Whether it's to make trade easier between nations, or to help our kids succeed in school, the outdated, common use of the imperial system has to end.
Part of the reason our children struggle with science is learning the Metric system and it's associated conversions.
At retail stores the litre is a commonly used unit for volume, especially on bottles of beverages, and milligrams are used to denominate the amounts of medications, rather than grains.
Because the system isn't completely new, the growing pains of change will be minimal. But most importantly, we do not want another Mars Climate Orbiter instance.