I attended a Louis Awerbuck class in 95 and he was working with a ghost ring made by Wickert. It was larger and thinner rimmed than all the subsequent ones I saw. Most were a standard rear sight with the aperture added on. The Wickert was a simple ring, and the bottom of the ring was almost on top of the slide.
I installed one on my Glock 19 and shot it for several years. I was satisfied with the (low) accuracy standard I was concerned with(I shot mostly inside 10 yards) and was able to get much faster. I think the speed increase was due to seeing more and accepting the coarse sight picture. Accepting more wobble meant less stabbing the trigger when the sight picture was "perfect". The result was that I could tear up the "A" zone with fast splits while I was shooting on the move.
After I started shooting USPSA and got exposed to more "advanced applications of the basics" (especially grip)and better training concepts, I was persuaded to move back to post/notch.
Awerbuck experimented with them because of his severe vision problems. I found most people who tried my setup very mentally or visually resistant to it, and did the equivalent of a "C-more shuffle" rather than shooting quicker.
In view of the development of Target Focussed shooting (and deteriorating vision)it might be interesting to try it again.