It's not just Microsoft
No matter what software you upgrade, you'll have some problems. It's rare if you don't, unless all you use is the applications that came with the OS, but who does that other than my Solitare playing mom? I myself will wait until my next computer build before going to Vista, just to be safe.
At work we upgraded our video editor PPC G5 to Leopard from Tiger, and I've never seen my boss so angry at a Mac. (he's a massive Mac fan who quotes Mr. Jobs daily) None of the Adobe software functions correctly, some won't even load, you can no longer access more than 10 computers at a time like you could with Tiger and a free piece of software, there's no iCal, it crashes, the interface changes that'll drive you mad, etc. Coming from Apple, where they say 'It Just Works'. Well, so far it doesn't. You could say it's Adobe's fault for not making their applications 100% compliant, or Apple's for rushing their operating system out to compete with Vista. So now we're left with either downgrading back to Tiger, or trying to reinstall every piece of software to see if that fixes the problem. And if any of you have installed OS X and Adobe's Creative Suite on a Mac, you know how long and boring that is. You have to sit there and verify every now and then that you want to install. I heard this was a complaint with Vista, funny that Mac's do the same thing.
With more people using Macs every day, and now with more hardware and software needing you to buy an Intel Mac (ZBrush, BluRay suport) they are not as upgrade friendly as their commercials make them out to be. Mac's also dropped classic compatibility, which is a huge pain for anyone that keeps archives of past jobs on old software, and can no longer access them without keeping an old Mac in the closet.
To sum it up, it's not just Microsoft, but any OS you buy that does a huge upgrade. There's always going to be someone that doesn't have a problem, and someone that does.
'I am enough of an artist to draw freely upon my imagination. Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world.'
- Albert Einstein