I've been running XP x64 on my home PC for quite a while now, and the only gotcha I had was the lack of TV Tuner card drivers (notably ATI All-In-Wonder). And drivers for other minor peripherals, eg webcams, scanners, etc. (If you have a SMB level printer from HP, Kyocera, etc, drivers are generally not a problem).
Application compatibility hasn't been a major issue for me personally, everything I've thrown at it worked, or was a 16bit application (which are no longer supported at all).
My new work laptop has Vista x64, and so far so good application wise**, just the lack of drivers for hardware again, which is the same as XP x64...
XP x64 SP2 feels snappier than XP SP2 x86, but then again that could just come from the hardware I have...
As for Vista, wait for SP1... Vista does have some good points, but IMHO the bad points currently outweigh the good points.
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** I did have to use Virtual PC to run Windows XP SP2 for some legacy Foxpro based applications, as only Visual FoxPro v9 is officially supported in Vista x86 only. (Visual FoxPro is not supported on x64 OSs).