I've got my backup server on FN11. No issues. I frequent the FreeNAS forums and while there have been the normal, small number of one-off problems reported, I haven't seen any widespread issues. I'm waiting to upgrade my main server until FN11.1, as that upgrades the jail system, and I don't want to rebuild my jails now, and then again at FN11.1. But, if you have no jails or VMs, I see no reason to delay upgrading.