On my new Athlon64 system I installed Gentoo 2004.0 using gentoo-dev-sources-2.6.3-r2 and choose xfs as file system for my root partition.
~36h later - while merging openoffice-bin - portage couldn't unzip some openoffice files to /var/tmp/portage. On my second attempt the same thing happened. 2-3 minutes later any attempt to read or write to the partition failed with "I/O error". After reboot the system hang at the attempt to replay the journal. I booted knoppix and tried to mount the partition but with not success either. After also xfs_repair from the knoppix cd decided to just hang, I reinstalled using ext3.
I'm sure that my HD is fine - it never gave me any trouble and a surface scan did not result in any errors. I'm running Gentoo on a Gigabyte K8VT800. Beside this incident the system never gave me any trouble under different operating systems (including gentoo with an ext3 root partition). Furthermore in the corresponding thread at the gentoo forums another user seems to have experienced the same problem, so chances are xfs support is not yet mature for the x86_64 platform.
I've removed the amd64 tag from the xfs-field in the installation instructions.
Can't fix, he moved on to ext3.