We've had a backup server running for a client with FreeBSD's ZFS for about a year now, also with zero hiccups. We first tried to get this kind of stuff working about three or four years ago, and went the OpenSolaris/ZFS route -- what a pain that was. OpenSolaris somehow corrupted its own boot partition one day and just completely refused to start with a totally cryptic error code.

The last time we tried Linux for this, it couldn't do it. Maybe that's gotten better though.

We also tried DragonflyBSD, but it had hardware support issues, and OpenBSD, but unfortunately OpenBSD just simply cannot do large filesystems. At all.

ZFS-on-FreeBSD is the way to go at the moment, I think.

ZFS on FreeBSD works a charm - I was happy to switch to FreeBSD after having done OpenSolaris & Nexenta specifically for ZFS.

I actually use ZFS on Mac OS, as well, and have been happy with it there (thanks Dustin Sallings!).