Hi!
Thanks for the feedback and the disscussion 
I know is a tricky question Group vs Layers but here are what I think:
Groups are not layers BUT could have the layer features, groups are more flexible and powerfull than layers and the user should have the freedom to use them as layer or as other things if they want to.
I have checked the layer manager of other packages as well and the other things they have are coloring, and several flags for representation like bounding box, and excluding from several rendering parts. that are of secondary priority rigth now
If we use Groups as Layers the main difference will be that layers group objects in an exclusive way, and groups in a non exclusive way, but nothing stop you from grouping objects in an exclusive way :rolleyes:
The issue regarding overlapping groups is simple: with ONE toggling always set the child objects of the active group to the same state of it, that way toggling TWO TIMES you can ensure there´s no hidden object that belong to other group and also that every object in that group has the state that you want.
I don´t know if some build has being made but seen that in work could fade some concerns.(I could not upload my build) I don´t see why that group manager could not replace a layer manager, they DO the same, whether easier or not is something that could be changed on the march, and on top of that agroup manager offer you the freedom to make overlapping and nested groups something that is unthinkable for layers but as stated before, one use case of it could be that “Superset” layer:
you have one layer_group for bolts and one for nuts, and you have a superset of bolts and nuts, that way toggling two times you could show/hide all bolst and nuts in your scene.
Off course, more things could be added, like state inversion and other minor features, and also I need to investigate the interaction with other parts of Blender like the current render layers if the aim is to implement the the 20 blender layers functionality, but for now is just a complement of the 20 layers for big set ups.
Currently I could not commit that myself and also need the review and the approval of other devs, so is not in my hands to integrate it, I just code it to attract the interest of some dev to integrate it.
I have to metion that although is not finished, that patch is pretty functional and usefull 
Cheers to all Farsthary