Io is an inhospitable moon of Jupiter. Though terraforming is almost out of the question, colonization of Io would be relatively easy using the geothermal energy to create an electromagnetic field so we could protect the colony from the radiation belt; allowing people to move about on the surface without wearing a lead-lined suit. But due to the volatility of the moon, the volcanoes that have been dormant for centuries could suddenly start flowing without warning and due to the lack of an atmosphere pyroclastic flows could envelop a person or colony instantly, thus we would have to make the colonies outer layers from tungsten with the inner layer made of asbestos. If we did put electromagnets in orbit space elevators would have to be made to transfer the geothermal power to the orbiting electromagenets. It is likely that Io will be colonized for geothermal power, it would be a great substitute for solar power this far out from the sun. Io is also the only of Jupiter's four major moons with a solid rocky surface, so it may be used for mining, though it is more likely smaller moons with less gravity will serve this purpose. However, while terraforming Io today seems nearly impossible, the fact that this would be the only moon of Jupiter with a solid surface may be a strong incentive for the people of the future to find a way to make it work.
Terraforming Io would likely only be possible if it was moved to be Venus' or Mars' moon. This can cause a reducing of volcanism, and intense solar winds can blast off the sulfuric acid. This process would happen automatically. After it happens, terraforming Io would be much easier. The gravitational tug of Io could help restart the internal dynamos of Venus, or Mars, giving them a much needed magnetic field.
Of note is that of all the moons in the solar system, only Io has a surface gravity stronger than that of Earth's moon, and has the most surface gravity of all the moons.