I bet few people ever used it, but I actually did for a short time. There has been an X11 / "regular Unix" implementation of #OpenStep besides GNUstep: for Solaris (Sun's OS at the time).
http://www.kevra.org/TheBestOfNext/NeXTProducts/NeXTSoftware/OpenStep-SolarisWhitePaper/files/page538_1.pdf
http://www.kevra.org/TheBestOfNext/NeXTProducts/NeXTSoftware/OpenStep-SolarisWhitePaper/files/page538_1.pdf
