The area of focus can look very sharp when scaled down, but usually it's a bit soft viewed at 100 percent. And it's small, too: I couldn't ever get both eyes in focus, even with tiny little kitten faces.
(1/100 sec, ISO 200, evaluative meter, AWB, Speedlite 580EX flash, Canon EOS 7D)
Even at f5.6 the depth-of-field is relatively small, so any slight shift of the lens or camera (in this case, the subject) can end up with the incorrect area of focus. Like the bridge of Potter's nose instead of his eyes.
(1/80 sec, ISO 200, spot meter, AWB, Speedlite 580EX flash, Canon EOS 7D)