I'm presently giving OS 3.9 a shot again. I have a 80Mhz 060 with 32Mb of RAM on it, but that didn't seem to help...
...at first.
Then I began to learn about the wide world of patches, modules and the like and I've been a happy camper since! Once you get the right patches applied, OS 3.9 really begins to shine.
I have always been a huge fan of Directory Opus 5.x as well. I would even run it even if my system was too slow, but now I can run it with OS 3.9 and my system still seems just as responsive if not more so than my initial install. Likewise many games and applications run better as well.
Of course owning an Indivision MKII probably helps a bit too.