According to Rowling's description, the Imperius curse is a lot like Heinlein's slugs: the controlled person keeps all their capabilities, but feels no worry or feeling of responsibility, and they want whatever the controller wants them to want. To make them more rational, the controller would essentially have to do their thinking for them. It's most useful for making someone else into your secret agent.

The thing I find most intriguing about the Imperius curse is that it's possible for someone under it to cast the curse on someone else. And several people can be controlled by a single Imperius-caster. This means that, if enough people could perform the curse, you could theoretically turn the entire world into your brainwashed slaves by setting up an Imperius tree.

Of course, your commands would be passed on through the imperfect interpretation of O(log n) intermediaries, where n is the size of the controlled population.