I keep a 32 bit OS (XP) running on a virtual machine, in case I need to to run some very old software that won't install/run on the latest Windows. But in general I would only use a 64-bit OS on my main computer, because fewer developers are supporting 32-bit versions of new software.
I've been running 64bit OS since release of Windows Vista. Yeah, that one. It was the first OS that had reasonable 64bit driver support. WinXP 64bit Edition was something I preferred not to bother with since support was pretty bad in general.