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.