So you never used 10.5.0 then or 10.6.0? both had major flaws. Leopard was terrible until 10.5.3. As for 10.6.0, I would call an upgrade that erases a users hard drive a minor bug, wouldn't you?
It was never that bad for the majority of users. The harder you dig, the less you'll find. There wasn't a massive movement to downgrade to the previous OS. With Vista, however, companies were actually offering to help users downgrade to XP. It was a total disaster.
Apple just went on selling more and more Macs each quarter. No change.
Vista was in a class by itself.