I used briefly on an emulator: System 6.0.8 & System 7.1
I also used on an emulator, as well as a real Mac: System 7.5 & Mac OS 8
I then went onto Mac OS 8.5, Mac OS 9 thru 9.2, then finally onto Mac OS X 10.2 Jaguar, followed by Panther and then Tiger.
I was always intrigued by the Macs, but they were always just for the "fun to play around with" factor. Indeed, when I bought my Beige PowerMac G3 DT 2nd hand in 2004, it was only bought to play around with the Mac OS and didn't become really useful to me, until I tried out OS X, albeit it was slow.
In 2 years I've gone from 2 Macs in my collection, to 8 Macs, 7 of which are running OS X. A refurbished PowerMac G5 Dual 2.0Ghz with 1Gb RAM, bought in 2005, is now my main system (needs upgrading a bit though, more RAM, bigger HDD and better graphics)! My next Mac purchase will likely be an Intel Mac mini, which would relegate my current G4 1.25Ghz Mini to some kind of Media computer.