One of the more common complaints I hear about with ZFS is when clients are using zvol's to offer up iSCSI or FC block targets to clients, formatting filesystems on them, and then using them. Well, I don't get complaints about that, so much.. what I get are complaints about how, over time, the 'used' space as visible in ZFS is completely out of whack with the used space as visible in the filesystem on the client.