I agree. With Windows, you don't really get a choice. The registry (possibly the worst idea, ever?) fills up with crap, software doesn't uninstall properly and so on. It's just easier to fire up Ghost and ten minutes later, I have a nice fresh XP installation, including Office 2K3, VS.Net and all my settings in place. If you do it as often as I do, it's not really that much of an inconvenience, though in ideal world you shouldn't have to do it (as you say). From what I've read and seen about Longhorn/Windows 20xx, I can't see basic tenets of the design - the Registry and the like - being improved to any great degree.
I'm not overly experienced with OS X (though I do want to get an Apple laptop (at least) at some point in the future to go with my iPod - I can't switch entirely because of programs like VS.Net (which is useful for work) and Chief Architect (which I love)) though I have used Linux which has some similarities to OS X. My distribution of choice was Gentoo, and keeping it up-to-date was fairly painless (though the compiling was a PITA). From what I can see from OS X, updating it is much easier and doesn't have to take as long. Which sounds great!
Incidentally, I don't mind the "Martha Stewart" UI in XP. The "Blue" and "Olive" themes are hideous, but the "Silver" one isn't half bad. And if you turn ClearType on, you'll get anti-aliased fonts like OS X. Windows is getting there, but slowly. Very slowly. Compared to previous versions of Windows, XP is a complete triumph.