I'm absolutely convinced that I observe a significant improvement in performance -- even when many applications are still 32-bit.  All of the various services and bundled apps with the 64-bit OS run faster.  I've seen bottlenecks opened wide, whereas XP 32-bit was choking my system -- or running significantly slower.  For instance, I don't have problems with the occasional stutter in video playback that would occur with XP MCE.  My TV application would halt momentarily when I'd open my e-mail browser.  There may be other aspects or explanations to that shortcoming, but it doesn't occur at all with my 64-bit VISTA OS.  And that's for comparing Media Center for each OS on the same quad-core system.
So far, there've been two software applications I've needed to upgrade for failure to run well in the 64-bit OS.  And I can no longer use my 13-year-old inkjet printer -- which had taken a licking and kept on ticking -- a workhorse.  I can still get the cartridges for that printer and get them easily.  But it has a parallel-centronics interface.  And the worst of it was the total failure of the driver bundled in VISTA-64 for that particular printer.
On the printer angle, it cost me $750 in 1996 dollars.  The replacement does everything as well or better than the antique -- and I picked it up last week for $220 in 2009 dollars.
I don't think I'll ever use a 64-bit OS again, at least -- for a workstation machine.  WHS 32-bit is fine -- for what it does.  But on the client/workstation end?  The only reasons to avoid a 64-bit OS are such as I've shown here, and  -- really -- it's about time to upgrade a couple $60 software packages and chuck a 13-year-old printer . . . . isn't it?