The main problem with US customary units is that they're defined in a length/time/force system, whereas SI is a length/time/mass system. This, basically, means that it's meaningless to convert from pounds to kilograms, or vice-versa, because the pound is a unit of force and the kilogram is a unit of mass.
The US unit of mass is called a "slug". If you've never heard of this before, you're not alone. A slug is a lb * s^2/ ft and denotes the quantity of mass that, when subjected to a force of 1 lb, will accelerate by 1 ft/s^2. You can also define something called a pound-mass (lbm), which is 1 lb / (32.2 ft/s^2), i.e. the force of a pound divided by the standard gravitational acceleration at sea level.
Not to be outdone, you can also define a unit called the kilogram-force, where 1 kgf = 1 kg * 9.81 m/s^2, i.e. force applied on a mass of a kg by the standard gravitational acceleration at sea level. Incidentally, the SI unit for force is the Newton, which is a kg * m/s^2 and denotes the force needed to accelerate a 1 kg mass by 1 m/s^2.
1 pound = 4.45 Newtons
1 kilogram = 0.068 slugs
Technically, when someone tells you that they weigh 70 kg, they really mean that they weigh 70 kgf. Or 687 N. Or why not 4.8 slug-force?
If you ask me, both systems are equally worthless. Most people avoid using SI prefixes on their units because scientific notation is so much simpler and more exact (200 GPa vs. 2 * 10^11 Pa or, even better, 200 * 10^9 Pa), so the base-10 advantage/disadvantage is moot. Furthermore, the moment you need any kind of precision in US customary, you immediately switch to decimal notation and avoid mixing units like the plague.
In fact, come to think of it, the only real problem with US customary is that people can't agree on whether they want to calculate things in inches or in feet, so you're always fucked trying to figure out which 12^n fudge factor you're actually supposed to use.
Proud member of the Canadian Broadcorping Castration