In school, I've used nothing but the metric system since, I would guess, the fifth grade (i.e, when I was about 10 years old). This includes high school physics as well as calculus, and also organic chemistry lab in college. As a result, I probably understand the metric system on a "practical" level better than the average American. (For instance, I know that a liter of milk weighs about 1 kg, and will fit into a hollow cube measuring 10 cm on each side.)I think that apart from an understandable desire to follow their own traditions people are generally averse to metric system because they think it's something incredibly complex.
Even so, it still seems a bit "unnatural" to me to use the metric system in everyday life.
Part of the reason is that, to me, metric units sound artificial and un-poetic, since English is full of proverbs and expressions relating to the Imperial units ("Give 'em an inch and they'll take a mile"; or "He's a 98-pound weakling"), but we've yet to develop any similar expressions based on metric units. (Okay, I can think of one -- "Millimeter peter", referring to a man with an extremely small хрен!)
Also, some Imperial units have the advantage of being derived from powers of 2, which makes it a bit easier to estimate measurements "by eyeball". (Dividing a square sheet of paper into 8 equal rectangles is easy -- you fold it in half, then in half again, then in half again! Dividing the same square into 10 equal rectangles is not so easy, because it's difficult to visually approximate one-fifth.)
Even so, I would certainly agree that, in general, metric names are much more logical and simpler to remember, and that it's easier to do calculations with metric units (especially in the case of physics problems where you have to inter-relate mass, distance, and force, for example).