Since this is in the Classic System and Software forum, I assume it's more about software than hardware. Then again, the two go hand in hand a lot of the time, as OS X has awfully steep system requirements.
I made the full-time move to OS X a couple years ago on my 450MHz iMac. The only reason I did was because Apple stopped updating Java for OS 9, and I really wanted to play Go on
KGS. That's it. If not for KGS, I probably would've stuck to OS 9 for another year or two. OS X was certainly a pain to use on that system, whereas OS 9 ran like a dream.
My father still uses OS 9 on our old Power Mac 9600/300 every day. It does what he needs. He uses iCab 2.9 for web browsing; I've shown him more "modern" browsers for OS 9 (like iCab 3 and Mozilla), but he sticks with iCab because it's just more usable. (Ever try viewing "modern" sites in a "modern" browser with a large font size? Everything overlaps, and nothing is legible. Stupid designers.) If he moved to X....what would be the advantage, exactly? We could spend hundreds of dollars on a new computer capable of running OS X decently, but it just wouldn't be worth it. It would make more sense to buy a used Power Mac G4 and keep running OS 9.
Sure, there's the occasional crash in OS 9, but it's just not a big problem. 8.1, 8.6 and 9.1 were all very stable (9.2 not so much, because Apple kind of stopped caring at that point and updated it only for Classic support).
I appreciate that I can go weeks without rebooting with OS X. I appreciate that I don't need to think about memory management. And I appreciate Exposé. But none of that makes OS 9 (or OS 8 or System 7, for that matter) any less usable. OS X just hasn't changed my life.