QA better than IR ?

A Question Answering (QA) system allows the user to ask questions in natural language and to obtain one or several answers. If compared with a classical IR engine like Google, what kind of key benefits QA bring to users and how to measure their distinctive performances. This is what we shall attempt here to determine, specially in providing a comparative weak and strong points table of each system, along with showing how QA systems, in particular our Qristal QA system, requires up two to six time less "user effort".