It was really amazing what could be done with PC/GEOS in all its incarnations on even the lowest-powered of hardware. It's to be expected, though, considering what Berkeley Softworks could pull out of a C64 with classic GEOS!
When I finally tried GeoWorks on a PC, I was stunned by it's performance and functionality. Unfortunately this was well past its prime since I avoided it for many years because of that initial experience.
That GEOS 64 was "barely usable" was more a reflection of what the Commodore 64 could provide than anything else. And I think more people than not recognized that fact. The problem with PC/GEOS is that it arrived at a time when Microsoft was releasing Windows 3.0 and trying to strangle competition in that space. Had Berkeley started working on it much earlier and got it out the door in '88 or '89, it might have had better success than it did.
Of course, at the time I was simply blown away by using something that looked almost as good as a Mac, so it's possible my experience (and my memory of it) is tainted.