My first 64 bit chip was the Athlon64 back in 2003 -- over a decade ago. If you're a developer in a compiled language, you presently either must (a) make a 32 bit version and ship it for both or (b) make separate versions and make yourself a support/testing nightmare. No surprise -- most developers opt for (a).
...but in a way, that makes using x64 Windows moot. Since there's no software for it (other than the OS itself a web browser or two), why switch? From extra registers, to more available memory, to the no-execute bit -- there's many good reasons to be using 64 bit software.
The real reason, of course, is that many business run ancient 16-bit applications that won't run under a 64-bit OS. This could be fixed with an emulator, but MS, unlike Apple, doesn't have a history of making backwards incompatible moves tha ultimately improve its platform.