My first exposure to REXX was Quercus Personal REXX for DOS in 1993. Anyone who has looked at Richard Goran's guide to REXX, or played around with REXXLIB.DLL, would have encountered it. In its day, it was the fastest one going, because it tokenised everything after the control-Z.
It came in flavours for DOS, OS/2, Windows 3.x and 95/NT. Of course, we were managing things like 4DOS and 4OS2 back then, on a freshly minted OS/2 2.10.
The language was somewhat more versitile than other languages like BASICA, in that it was more 'open' to the OS. You could for example, edit binary files, and make decisions on what part(s) of the binary to edit. Back in the day, I was collecting operating systems as they hit the deck.
Quercus REXX is the only rexx that I replaced OS/2 rexx with. Some programs did not like it (eg GearOS2), but because of limited memory and such, Gear (a cdrom burning proggie), was run in an OS/2 PM session, with filebar as the shell.
Looking at the feature-set, Quercus is closest to Regina, except that it is not ANSI complient (no changestr), but was getting sufficiently close to OS/2 that they were advising people to use the OS/2 functions where appropriate. Some of IBM's commands were slightly supperior (eg FILESPEC, DIRECTORY).
Some of the early AI stuff (like renaming OS/2 icons and attaching help pages through the MINDEX object), were done in this. Using Norton Utilities 'InstDOS', thing, one can create whole Windows/WinOS2 desktops through rexx. I still do it that way.
PC-DOS 7 had a version of REXX for DOS. It caused a few headaches with 4DOS, because it used REXX ,BAT scripts.
When the 486 finally died in 2002, I moved across to Windows 98, Windows 2000, and finally (last year), Vista 6.1. None of these had REXX, but you could coax regina or even quercus to run in these environments.
4NT will see and use rexx .cmd programs, but i normally pass these through as .rex scripts.
Regina Rexx is about the only ANSI level REXX (non-OO), and being on DOS, OS/2 and NT, is pretty versitile passing batches through. The recently posted RXLOOP.REX works just as well under OS/2 as under NT (where it was written)
All of the rexx scripts handle bignum. Have a look at
http://zabrodsky-rexx.byethost18.com/aat/index.html?i=1 and my fibonacci series calculator (it's the world record, written under rexx).