Anyone wondering what the verdict is 3 years later? I am seriously ticked at my 64-bit system. It serves a dual purpose as an HTPC and general computer (no gaming.) If you're heavily into Media Center you can't do half the things you could in Win7 32bit. It won't play mkv, iso, zip or many avi files without serious tweaks and hours of filthy words thrown at it.
It also does not take kindly to registry tweaks, particularly those from .reg files. So all of my good old XP tweaks that worked beautifully in Win7 32-bit need to be hand converted 1-by-1 to add the extra matching 64-bit keys. Batch scripting .reg files is a no-go too.
Sandboxie isn't even worth using in 64-bit because you have to disable most of the security it provides to get it working.
IE8 in 64-bit version isn't worth opening. It will often refuse to work or just crash. I couldn't even get through a common browser benchmark. Not bothering with IE9 because of rumored problems.
Most browsers plugins don't work with 64-bit (Gecko and Webkit included,) or they have beta versions that, once again, crash. Although, no problems with Java 64-bit yet.
Most programs are still 32-bit. The ones that offer 64-bit versions make you install, and often run, the 32bit version right beside it. (I have 5 (out of 154) 64-bit programs installed right now. So much for NEEDING 64-bit.)
You can have more ram -- and it REQUIRES, not only more ram, but more hard disk space, more virtual memory, more cpu power, more cooling, more everything...... and I mean A LOT more. I've actually seen Chromium eat 6Gb of ram by itself (I'll stick with Firefox x86 thanks.) My Windows system folders are more than double the size of my 32 bit system (NOT including the page file, which is on a different disk. Hibernate is off. Thank goodness tech is cheaper now.
Virtual machines are nice... I could do that with PAE in 32-bit though. Not a bonus.
Better security? Microsoft's definition of security is, prevent the user from screwing up his computer so he doesn't cost us more money in tech support. Win7 64-bit is even more locked down than 32-bit. My computer, my screw ups, Microsoft. I know how to backup! However, documentation might help a little.
One thing does run better: MPC-HCx64 plays HDTV (.wtv files) much more smoothly than MPC-HCx86. However, 32-bit Media Center plays them just fine, and much better than MPC-HC so I'm not impressed.