In the US (and just a handful of other backwards places), children are generally taught the imperial units and nobody seems to know very much about the metric system at all.
In most of the rest of the world, children are generally taught metric units - metres, kilograms, litres and so on, and the imperial system (inches, pounds, gallons) if mentioned at all is generally derided as being an outdated, overly complex system. A teacher at school once gave me a hard time for measuring out a circuit board in inches - I explained that I had done it that way because the spacing between the legs of the microchips I was using is one tenth of an inch. That shut him up, though he seemed quite surprised to encounter these old fashioned units in such a high-tech context.
A few years ago it became the law in the UK that most things that are sold by weight or measure must be measured in metric units rather than imperial units. Not everyone was happy about this.
My opinion is that all children should be taught both systems of measurement - it is an important skill to be able to convert between different units, and it helps with mental arithmetic. Also, physicists often use non-metric systems of units, setting one or more of c (the speed of light in a vacuum), G (Einstein's gravitational constant), hbar (the reduced Planck's constant), the Coulomb force constant and k (the Boltzmann constant) to 1 to simplify the equations. These natural units are arguably much more fundamental than the metric units and their values (while not generally very good for day-to-day use) give important insights about our universe. The metric units aren't particularly fundamental - the metre and the kilogram are based on (inaccurate values of) the size of the Earth and the density of water respectively.
Like natural units, the imperial system is also better for some things (many people find the units more convenient, especially for cooking) and isn't completely going away any time soon. While having multiple existing systems of units has caused problems in the past, software can keep track to avoid this sort of thing as long as units are always specified (and they always should be, even in the metric system, as one could get confused between grams and kilograms for example). Accuracy of the older imperial units isn't a problem as they have all long since been redefined to be exact rational multiples of metric units.
I also think that people ought to be allowed to sell things in whatever units they find convenient, as long as that value is either a recognized standard or the conversion factor is clearly posted.
The one problem with some imperial units (in particular, those for measuring volumes) is that they aren't standardized across the world, as I discovered to my dismay when I moved here and first ordered a US pint of beer.