I'm currently running a backup box nearly identical to the OP, and haven't had a problem yet (built it over a year ago). ZFS runs like a dream on Ubuntu, even with a slew of oddly sized disks (1TB + 2x2TB + 3TB) at 90% capacity. I've had my SATA card come loose, and ZFS just locks down the FS to r/o so no damage is done. And unlike many other file systems, ZFS's checking utility actually gives you human readable results if there is an error (eg. "/foo/bar is corrupt", not just cryptic messages), and outputs exactly what you should do to repair data.

I recommend at least trying ZFS out in a VM, I guarantee you'll be impressed by the versatility.