Jan van Leyden said:
The main reason for the metric system is, imho, the weird relationship between the imperial units. Twelve inches make a foot, three feet make a yard, how many yard make a furlong, a mile, a statute mile, a nautical mile?
How do you convert square feet to square miles?
In the metric system, the name of a unit makes clear its relationship to related units. And I'm very much for systems I don't have to learn by heart.
I am not sure that this is a valid criticism of the Imperial system, actually, at least when talking about day to day life...
The value of the Imperial system is that each unit is ideally suited for a single task. Measurements of roads and long distance are made in miles, and shorter distances are measured in feet or yards, so there is very little need to ever use the 5280 feet = 1 mile conversion value in daily life. Human height is very easily measured in small values of feet, such that you can easily know that 5 feet = shorter than average, 6 feet = slightly taller than average, and 7 feet = basketball player, without even looking at a more precise breakdown.
Similarly, the acre is a great unit for measuring land, and while the conversion between acres and other units can be nightmarish (1 acre = 43,560 square feet, 1 square mile = 640 acres, though it makes a lot more sense if you know about the more obscure chain and furlong units), most measurements of land can easily be made with just acres, and conversion is not important.
I guess to turn thing around... when is it ever necessary in daily life to ever make the conversion between square feet and square miles?
For the units in which conversion is actually necessary, like the closely related inch and foot, or the pint, quart, and gallon, conversion is made using intuitive numbers like 2 and 12, rather than the unintuitive ten. Ten may be the base number of our mathematical system, but it is not very good for division and multiplication of substances without measurement (it is a lot easier to pour a gallon of water into two even half-gallons, and divide those into four even quarts, than it is to evenly divide a liter into ten deciliters without measuring).
SI units are great for complex mathematical calculations and plugging numbers into equations without using conversion factors, but they nowhere near as good for day to day life. I would never use miles and pounds when calculating force and energy, but I hate using SI units for basic things like height and weight in my normal life. Imperial units have a big advantage in how they evolved over hundreds of years through being used by normal people, rather than be imposed from on high by some guy who thinks he knows better like the Metric system was.
Edit: Regarding the whole "The US has already officially converted to metric" thing. If the US Federal government can't try a citizen for murder, give someone a driver's license, or give legal status to a marriage, then I say it simply doesn't have the authority to tell people what units of measurement they should be using.