Just a quick note - a new Path Guiding branch build has dropped (cheers Sebastian) which now includes techniques that have glossy guiding as well as diffuse.
Some tests are being posted here - but early results look promising.
edit - the algorithm has been updated to include a user editable glossy threshold, which should improve guiding for glossy materials with lower levels of roughness.
With glossy guiding
Without glossy guiding
(note these are intentionally noisy test renders to help highlight differences, rather than show off the full potential).