We're using ZFS on our remote backup servers, using 64-bit FreeBSD 7.1. Runs quite nicely. Requires a bit of tuning of /boot/loader.conf to get it to be stable, and how you tune it depends on the amount of RAM you have. But once it's tuned, it's solid and stable.
Our servers are 2x dual-core Opterons with 8 GB of RAM, and 24x 500 GB SATA disks in raidz2 with / and /usr on CompactFlash. Doing rsync backups of 85 servers every night, snapshotting after every run, and rsync of the snapshot to a remote server for off-site backups. With a (very) non-optimal raidz2 layout (24-drives in one big raidz2), we're getting 120 MB per second for reads and 100 MB per second writes.