Very, if South Korea is any indication:
Life has gotten much better for Koreans as the country has gone from 28 percent urban in 1961 to 83 percent today. Life expectancy has increased from 51 years to 79—a year longer than for Americans. Korean boys now grow six inches taller than they used to. South Korea's experience can't be easily copied, but it does prove that a poor country can urbanize successfully and incredibly fast.
The Dish 