I'm using XP x64 build 1218 - mainly because I didn't want to have to buy XP for my new PC, only to pay again for an upgrade to XP x64. I figured that if I couldn't live with the bugs, I'd just buy and install 32-bit XP, and wouldn't have lost anything.
I'm still waiting for a scanner driver, the printer driver can be a little flaky, I've had to buy an ADSL router because my USB ADSL modem didn't have drivers, a couple of games have refused to run, and several programs needed to be in the DEP exceptions before they'd work. The first x64 compatible on-access virus scanner was only released a fortnight ago, and Windows Update doesn't work, so the only way to update the OS (AFAIAA) is to download the latest build (I think they're on 1280, with 1288 coming soon) and reinstall with it.
Apart from that, it's been fine, and has saved me £80. If you want to find out more, I suggest you check out the forums at PlanetAMD64