I have zero problems, except for my audio drivers.. I have an nForce 3 board, but the audio isn't nforce-based, so there's no x64 driver yet, so atm I have no sound.
But C-Media said they'd have beta drivers in Feb.. so hopefully I'll see some soon.
Anyway, everything seems the same, haven't had one 32-bit app I use not work (Nero, DVD Decrypter, xchat, gaim, Firefox, Thunderbird is about all I've got installed atm).
32-bit apps get installed to x:\Program Files (x86), and 64-bit apps go to x:\Program Files, atm I only have one 64-bit app, which is sysinternal process explorer (handy tool for killing file handles that programs leave behind)
It's not really a 64-bit version of WinXP, it's actually based on Windows 2003 SP1, so it's fairly stable and works great, and supposedly has more optimized file handling/read/writes and such. Although lots of software doesn't have 64-bit counteparts yet, they're coming and most 32-bit versions work great.
If you're lucky and all your hardware has x64 drivers, give it a try, if not, just wait a couple months, it's like the old days of Win2K, which I was an early adopter of as well, even tho it made my modem connect at 33K
