Yeah, it definitely looks like you've got a bug somewhere. For comparison, here's some images from my ground-truth renderer, showing roughness values starting at 0.01 and ending with 1.0:
StPeters_Small.png 299.17KB 0 downloads
Uffizi_Small.png 364.26KB 0 downloads
FYI these are taken with an exposure of -2.5, which is a linear exposure of 0.176. It also has filmic tone mapping applied after exposure, followed by gamma correction.