Many thanks for this! Sorry for the delay. I hope to be able to have a look in the coming week.

I have tested a quick import and it seems to work great! See attached import.

It also seems as though we have both figured that Blender's collections feature does a great job of spatial decomposition! See: https://github.com/IfcOpenShell/IfcOpenShell/tree/v0.6.0/src/ifcblenderexport

If @aothms is OK with merging this, I'd be happy to do so, along with test it on a real project that we're doing in Blender right now :) And perhaps implement more tricks such as reusing mesh instances, and an option for putting the IFC class in the name as well as a custom property (it isn't necessary, but we found when it helps modelers stay organised and navigate the outliner)

If @aothms is OK with merging this

Yes, yes, yes.

And perhaps implement more tricks such as reusing mesh instances

There is a bug related to that. #661 https://sourceforge.net/p/ifcopenshell/discussion/1782718/

And perhaps implement more tricks such as reusing mesh instances

Looking at code it seems like it does that when loading (or am I missing something?)

    # MESH CREATION
    # Depending on version, geometry.id will be either int or str
    mesh_name = 'mesh-%r' % ob.geometry.id
    if mesh_name in bpy.data.meshes:
        me = bpy.data.meshes[mesh_name]
    else:

edit Just a bit slower than @aothms. That's a good bug. I haven't thought about it, but I think I could easily fix it.

IFC class in the name as well as a custom property

I was wondering if that would be helpful. This is the first time I am exposed to IFC files at all, so I don't really know all the quirks and relations you can store in that file.
I was wondering how much information is lost and whether it would be possible to pull out all data that IFC file contains so that you could re-save it without losing any data (or ideally without changing internal structure at all).

From other things I was also thinking about adding extra level collection geometry.
Currently I found it really annoying when I loaded a model of house that IIRC I couldn't easily hide ceiling bits as they were in structure above collection with walls.
I could make that change before this gets merged (or after, don't really mind).

Also cherry picked the commits into v0.6.0 branch.