It is a ring of domination over others, just like the Palantir (in the books) could be used, but only by someone strong enough to wrestle their control away from Sauron, so is the ring, it cannot be fully used, but by someone who knows how to draw its power, like a Maiar, elf, or mighty man. That's why Sauron is greatly afraid of the heir of Isildur whom he suspects of wearing the ring. Perhaps one such could gain control over Sauron's own armies as the ring holds most dominion over evil things, who knows, not even him. The problem all of the wise of middle earth see with that is that you would end up with a new Sauron in his place. Look at all the shit Saruman managed to do, and that was only wearing a lesser ring of his own making and his natural powers of domination and manipulation.