The thing I'm finding great about the 15-f/8 body cap lens is that it's great for street-style photos in good light, especially having a detent for the hyperfocal focus distance. Most images are adequately focused at all distances I'm concerned with, and if I force my G5 to use a high enough ISO, the resulting shutter speed is fast enough in bright light to reduce motion blurr to a minimum.
I've been comparing using the 15-f/8 with the Lumix 20-f/1.7 for street-style grab shots, using the G5, where with the 20 I either have too wide an aperture (and thusly too narrow DOF for quickly-taken grab shots) or the shutter speed is too slow to reduce motion blurr. I've taken to forcing the G5 to a higher ISO, then using Aperture priority mode select f/5.6-f/8 aperture, but still have to manually set the hyperfocal focus distance in manual focus mode, which is hit or miss because of the 20-1.7's lack of a manual focus scale, and the G5's lack of focus distance information in the EVF.
In this regard the 15-f/8 just performs better as a street-shooting lens for quick grab shots, because its slow aperture forces the G5 to a higher ISO, resulting in a faster shutter speed, and the hyperfocal detent is just more accurate than my hit-or-miss focus preset method with the 20.
As for image quality, yes, the 20-1.7 is sharper than the 15-8, but I use a geometric and CA correction development style in SilkyPix that improves the resulting files.
I have grab shots from a recent vacation to San Diego taken with both lenses, I'll have to post some results soon.
~Joe