I don't think ClearType gets enough credit for being as impressive as it is. Compare these two screenshots of an e-mail from OS X and Windows 2003, and tell me which you prefer. (It's interesting to note how much larger the text is in Windows, too, given that the site is specifying both the font name and the text size.)
E-mail in OS X
E-mail in Windows 2003
Update: By request, here is the same e-mail as viewed in Vista Beta 2. To my eyes, this is the best of the three. Oh, and I'm running Vista on my iMac after following Jeff Dorsey's directions (as related by Hans). Finally, an anti-SUD! :)
E-mail in Vista