I actually started out using the Acorn BBC Model B, graduated to an Acorn Archimedes, then moved to RM Nimbus, which was sort of DOS, but not quite, then we moved to PC, first on MS-DOS, then gradually shifted up the varieties of Windows to the latest computer that arrived yesterday for G, which runs Vista.
Along the way I've also used a couple of palmtops (a Sharp, I think it the first one was and then a Psion 5MX) and now have an Eee. So in fact about the only OS I haven't used is the Mac.
I've never quite understood the Mac hype. Yes, perhaps they are better, but they're that much more expensive and on the odd occasion I've played with one, I haven't found them easier to use. Mac fans claim they're more intuitive, but that's not really true. No computer is intuitive until you've used it for a while. I know that because I've taught hundreds of beginner adults to use a computer. If something was truly intuitive, then I could have sat a complete novice down and just let them get on with it. But that just will not work. Using a mouse is not intuitive, you have to show a beginner how to move it and how to click. Of course once you become familiar with a computer, as long as a new piece of software runs in the same way as the ones you've used previously, then it appears to be intuitive, but that's not the same thing.
Regarding the supposed superior reliability of a Mac, I have never replaced a computer because it stopped working. It's always been because it's become so outdated that the software I want to run or the gadget I want to connect to it no longer supports that OS. Paying more for something that would last even longer seems somewhat pointless.
But I do like my iPod. :)