Actually, I stand corrected. I just tried taking the same picture twice with my F717, once with full optical and digital zoom, and once with full optical only. Resizing the digital zoom picture to 25% 640x480) gives just as good a result as cropping the optical zoom picture to 50% (1280x960) and the resizing that 50% (640x480). So 2x digital zoom is somewhat useful, and can be used as a shortcut to cropping an image.
But, larger digital zoom factors are bad. With a 3x digitally-zoomed image from my 2-megapixel P31, I have to resize to 25% (down to 400x300) just to get an unmagnified image. And, this camera is far from sharp at full resolution. The picture has to be resized all the way down to 12.5% (200x150) in order to look decent. That's practically a thumbnail. But, it's really not a fair comparison, because the two cameras on on opposite ends of the spectrum.