Joel did a great job of explaining why, when it comes to actually getting work done, "Windows XP is a better experience". He mentions a few specific things, like the WiFi locking up, fonts being blurry, and feature support.
Now I haven’t used a Mac for day to day work, but I have used Vista RC1. The fonts look even better. Aero is beautiful and useful. There are a few things I prefer the way Mac did them – like dashboard remaining hidden until you press a key combination and then hiding again. I hate the sidebar! What a distraction. I like the gadgets, but disappear, please, except when I want to look at you. But these are minor. And I wouldn’t trade them for the fonts and autocorrect :).
Now the new network stack does seem a little flaky to me – it does hang sometimes for me, nothing like Joel has seen. And it has improved dramatically with each build (Beta 2, Beta 2 refresh, RC1). And I suspect a clean install might have cleared that all up. I think I’ll wait for the clean install though until Vista ships.