Impact drivers will actually start a screw faster, drive it in further, pull tighter with the screw. Perfect for installing metal roofing panels, wood subflooring, anywhere a rough driver with limited control will work it even seems to give you a control over the depth as it 'ratchets'. Works with a predrilled hole fine.
For fine woodworking, leave the impact driver alone.
Compared to an air powered driver.. less power, but no air hose.