I'm running a ZFS NAS with mirrored 2 HDD vdevs. I've set the HDDs to spindown after 20mins using hdparm -S and this works fine under zfs. The drives (Toshiba N300s) take ~15s to spin up when something accesses them, and ime zfs has always handled this gracefully.

This vdev only contains data; it's not running the OS / root.