Somewhat related anecdote in light of 10.8: I never properly upgraded from OS 10.5. I pirated 10.6 and skipped 10.7, partially because I'm a cheap college student and partially because the changes between versions never seemed that dramatic to me.

I had to re-install my OS and thus downgraded from 10.6 to 10.5. All of a sudden, none of my iLife apps are working, plus a host of others, and I can't download Apple ones designed for 10.5. I lose momentum scrolling, and realize that, hey, an upgrade once every three years is a good idea. I wait for 10.8 to come out, so I can buy it like a good, normal consumer, and soon discover I need 10.6 to install it.

So I'm buying two upgrades? Alright...wait, I can't buy 10.6 anymore. 10.8 is the only OS available, and I don't have 10.6 on a disc. So it looks like I'm pirating again.