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
@helge I played with it briefly back in the day, but at least for me there was no reason to use it. Running NEXTSTEP directly on SPARC was a much better use of a SparcStation.
@gregtitus Well, we’ve been deploying on Solaris a lot (Netscape Enterprise Server etc), so that was actually a pretty good setup to work on. Expensive though 💸