This is the part where, as a cross-platform user, I have to point out some contrasts.
I have upgraded OSX on my four-year-old iMac three times since I got it. Not a one of those upgrades was anywhere near as troublesome or clumsy as upgrading from XP to Vista.
I didn't have to download an external advisor, almost all of my applications and games worked without a hassle, and I haven't encountered too many graphics limitations just yet. I didn’t find any limitations to the number of times I could install it on my household Macs either, but I only have two at home and one at work. (I know there are limitations.) I upgraded all three without encountering any user limitations.
Now, I'm not preaching for either side. I love Vista on my new computer. It runs well and everything works somewhat smoothly, but upgrading seemed like such a hassle when compared to the same procedure on a Mac. Given, there is a far greater difference in the operating systems and my argument just is not fair, but as a tech enthusiast myself, I couldn't help but mention this.