Well, at the risk of sounding like just another Windows user who's switching, I'm seriously considering a switch. My situation is slightly different from the "average" user, in that I'm an ASP.NET / Oracle / Sql Server web developer and consultant.
Thanks to Parallels I plan to run a virtual XP machine with Visual Studio and SQL Server for development, but "live" in OS X for general use. The way I see it I get
(a) better hardware for a (small) premium (I'm replacing a Dell laptop, and we're only talking £150 or so for a MacBook over the equivilant Dell)
(b) a better OS to "live" in.
(c) the ability to test my web applications on all major platforms (the only way to test in Safari is through Mac hardware, to my knowledge)
(d) something new and cool to play with :-)
As more and more programs are being developed as web applications (c) actually becomes a pretty compelling reason to make the switch for a developer. It'll be interesting to see how it works out, as I certainly won't missing "living" in XP, even though I am stuck with it to make a living.
Windows: Never bad enough to sell Macs by Default