is/was really sweet. Variants of it ran on the Color Computer
and many CoCo
owners I knew swore by it.
They had things like interlaced GIF display while downloading years before anybody else did.
I had the pleasure of working with OS-9's successor, OS-9000 (I'm not sure what the differences were between OS-9 and OS-9000 other than a larger memory area and more modern underlying memory management).
OS-9 also ran/runs in various forms on the Atari ST and I kept hearing about an Amiga port but never actually saw the beast.